How do I perform a find in a related database?

It is possible to simply search on related fields, but the results can be unexpected. See the "Find in portals" article in the Articles section of this forum.

Searching the other file requires a script in that file, and some global fields either in the current or related file. So, you would first enter your find criteria into the global fields, then call a script that calls the external script, which goes into Find mode, sets the fields to the vales of the globals, and performs the Find.

The best answer to your question depends in part on whether you are serving your solution as a peer-to-peer solution or as a web solution, which you did not specify.