How can grant permission to execute a stored procedure on another database to specific master data services user?

Question

Problem:

I need to grant permission to execute a stored procedure on another database to the master data services user that is handling the DML commands whenever I use the Master Data Services Excel add-in.

Context:

I added a SQL trigger to a master data services table. Whenever DML is used on the table the trigger need to execute a Stored Procedure on another database.

The user initiating the DML will be using the MDS Excel add-in.

However, an error is thrown in Excel whenever I click Publish (where data is pushed to the MDS db):

The server principal "S-1-9-3-4000979447-1289397781-4196995230-646085745" is not able to access the database "IHS-Dataplatform" under the current security context., @ErrorNumber = 916, @ErrorProcedure = "dbo.ExecuteWhenTriggered", line 57

I can't find the server principal anywhere, hence I am not sure how I can give the access to execute on another database.

I tried to have the trigger execute a Stored Procedure on the MDS database itself. In order to do this, I needed to "GRANT EXECUTE ON" the Stored Procedure to the mds_schema_user. However, I can't grant execute on a stored procedure on another database as this user is a sql user without login.