How can I add custom dimensions to a Universal Analytics tag in a server-side GTM container that receives its hits from a GA4 tag?

Question

This is my situation:

• A client-side GTM container contains a GA4 tag that sends its hits to a server-side container containing custom event parameters, e.g. page_type
• A GA4 client in that server-side container that receives these hits
• A Universal Analytics tag (among others) in that server-side container that forwards the hits

How can I add the value of page_type to the Universal Analytics tag as a custom dimension?

Answer 1

It turns out you can use x-ga-mp1-cd1 in the “Override Fields to Set” to achieve this, as mentioned here by Simo Ahava.