IBM Targetprocess

I'm trying to build a mashup in Targetprocess that adds a multiselect dropdown field called Predecessor to ProjectTask entities.

Here's what I want:

• The dropdown should show all other ProjectTasks that belong to the same TraditionalProject.

• It should exclude the current ProjectTask itself (to avoid selecting itself as its own predecessor.  

Any help, clarification, or examples from others who’ve done similar would be appreciated! I've attached what I've tried so far.