How can I set the position field on the assignment screen up to be required at data entry time?

I need to set the position field on the assignment screen as a required field.  I have gone to the assignment form and chose <Help><Diagnostics><Custom Code><Personalize>

From here I defined the following fields on the Condition form

Seq - 10

Description - Position_Required

Level - Form

Enabled - checked

Trigger Event- When-validate-record

trigger object - assgt.

Processing Mode - Both

Level - User

Value - my userid

On the Actions form, I defined the following fields:

Seq - 10

Type - Property

Description - Position_Required

Language - All

Enabled - Checked

Object Type - Item

Target Object - Assgt.Position_Name