How to use WTUser Variable in Partcipient Tab of an Workflow Activity ?

Hi All,

Is it possible to assign a certain workflow activity to a user which is selected by a user while creating a Windchill Object.

I found Variable option under Participant tab in Activity robot. I have created a "WTUser" type of variable in workflow, still below pan shows blank.

Please guide me to use this functionality.

Regards,

Vivek