Validation Rule on Person Type in Add a Contingent Worker Flow Redwood

Summary:

At this moment the client as an Auto Complete Rule which will trigger when anyone will do the Add Contingent Worker flow on Person Type. They namely have set up:

• AAN - XX
• EXC - XX
• EXT - XX
• EXT - XX
• EXT - XX
• EXT - XX
• EXT - XX

Everything that starts with ''EXT'' should not be selected in the Add a Contingent Worker Flow, because it can only be selected in the recruitment process. They have built an ACR for this, but that one doesn't trigger in Redwood. 

I wanted to built a Validation Rule in Redwood and now how to do it. However, I cannot select the correct field to do this. It is namely referencing to ''User Person Type ID'' as a field, but I can only select ''Proposed Person Type'', ''System Person Type'' and ''Person Type''. All which do not do anything as a validation.