This article explains how to use advanced search to filter Jira issues based on values on Multi Project Customfield
Instructions
To build an advanced search
Click on Issues on the top navbar, then click Search for Issues
Click on advanced
Write the custom field name and follow it by space
User is able to pick from the following operators ( is , in , = )
An autocomplete drop-down shall show up with your projects
Basic JQL search will be available once forge support it in an upcoming release.