Filter search box using other list

I was using ProjectSuplier as source on the Searchbox of Activity. But wanted to store the Supplier in a specific supplier field on the Activityist. Furthermore I als want to prefill the current value of the searchbox (ProjectSupplier) based on the value of suplier. That seems not to be possible.

So now I think I need to just link ProjectSupplier to Activity.

Any thoughts how to remove duplicates? I have managed to maintain a unique list of SelectableProjectSuppliers. Maybe I need to link that list to Activity. On the background I believe I can sync Activity.Supplier field to be able to manage the privacy rules on activity.

I will try something out and get back to this topic.

Thanks!