Email signatures for employees using Google Workspace

Question

I'm developing a web app that allows you to create email signatures for your company. The idea is that once you have your email signature, I want users to be able to click a button and install those signatures to their team members that use Google Workspace.

I've seen other apps achieve this by allowing the Google Workspace admin to install an add-on (from Google Workspace Marketplace) and giving permissions. The add-on has no UI, but it then allows their web apps to install the signatures into each employee.

How can I do this?

I have tried to achieve this by enabling in Google Cloud the Gmail API, Google Workspace Marketplace SDK, Google Workspace Add-ons API, Admin SDK API (among others), and creating an add-on for this. But it doesn't work because I get lost on how to to make my web app effectively send and install the signatures to the Google Workspace employees.

I have also created a JSON deployment, but don't know how to "connect" my app to Google Workspace:

I have also tried to create a service account to get domain-wide delegation (?) but this didn't work.

I have already implemented Google OAuth for user signup/login (with Supabase) and importing contacts from Google Workspace to easily invite your team to my app.

src/routes/api/workspace/signatures/batch/+server.ts

import { json } from '@sveltejs/kit';
import { google } from 'googleapis';
import type { RequestHandler } from '@sveltejs/kit';

export const POST: RequestHandler = async ({ locals: { supabase, user }, request }) => {
    console.log('1. Starting batch signature update');
    
    if (!user) {
        return json({ error: 'Unauthorized' }, { status: 401 });
    }

    try {
        const body = await request.json();
        console.log('2. Request body:', body);

        const { teamId } = body;
        if (!teamId) {
            return json({ error: 'Team ID is required' }, { status: 400 });
        }

        // Get team members
        console.log('3. Fetching team members for team:', teamId);
        const { data: teamMembers, error: teamError } = await supabase
            .from('team_members')
            .select(`
                company_members (
                    user_id,
                    users ( email )
                )
            `)
            .eq('team_id', teamId);

        if (teamError) {
            console.error('4. Team members fetch error:', teamError);
            return json({ error: 'Failed to fetch team members' }, { status: 400 });
        }

        if (!teamMembers?.length) {
            console.log('4. No team members found');
            return json({ error: 'No team members found' }, { status: 400 });
        }

        console.log('4. Found team members:', teamMembers);

        const gmail = google.gmail({ version: 'v1', auth: new google.auth.GoogleAuth() });

        // Update signatures
        for (const member of teamMembers) {
            console.log('5. Processing member:', member);
            
            const userEmail = member.company_members?.[0]?.users?.[0]?.email;
            if (!userEmail) {
                console.log('Skipping member - no email found');
                continue;
            }

            console.log('6. Fetching signature assignment for:', userEmail);
            const { data: assignment, error: assignmentError } = await supabase
                .from('signature_assignments')
                .select('custom_data')
                .eq('assigned_to', member.company_members[0].user_id)
                .single();

            if (assignmentError) {
                console.error('Assignment fetch error:', assignmentError);
                continue;
            }

            if (!assignment) {
                console.log('No signature assignment found for:', userEmail);
                continue;
            }

            try {
                console.log('7. Updating signature for:', userEmail);
                await gmail.users.settings.sendAs.update({
                    userId: userEmail,
                    sendAsEmail: userEmail,
                    requestBody: {
                        signature: assignment.custom_data.html
                    }
                });
                console.log('8. Signature updated successfully for:', userEmail);
            } catch (error) {
                console.error(`Failed to update signature for ${userEmail}:`, error);
            }
        }

        return json({ success: true });
    } catch (error) {
        console.error('Error in batch update:', error);
        return json({ 
            error: 'Failed to update signatures',
            details: error instanceof Error ? error.message : 'Unknown error'
        }, { status: 500 });
    }
}; 

I guess what I need the most is guidance on how to achieve what Scribe does here: https://help.scribe-mail.com/scribe/install-signatures/automated-installation/google-workspace-gmail (other apps like WiseStamp, Exclaimer, among others, do this too). Like which tables should I create in Supabase for this, how should I create the add-on in Google Workspace, how should my app communicate with the add-on to install the email signatures to all company members, and so on.

Thank you in advance 🙏

My tech stack:

• Svelte (v4.2.13)
• SvelteKit
• Typescript
• Supabase
• Vercel