If this is a Catalog Item, the easiest way to accomplish this is with the new (Utah and later) variable Auto-populate feature.
https://www.servicenow.com/community/developer-articles/auto-populate-a-variable-based-on-a-referenc...
If this is not a Catalog Item, or you are on a pre-Utah version, you'll need to use an onChange (Catalog) Client Script with getReference
https://docs.servicenow.com/bundle/tokyo-api-reference/page/app-store/dev_portal/API_reference/Glide...
