List of Values search populates all project activities

I have a project set up that has related work activities.  Note that this activity field is a custom field.  When I want to search for an activity to attach to this particular project, the search brings up every activity in the database, for “all” projects.  Is there a way to customize this activity field or the Search to only populate the activities for the project I have assigned in the line?

Just looking for a way to possibly speed up the search.