TFS Changing "Assign To" list according to State field selection

TFS Changing "Assign To" list according to State field selection

In TFS Work Item Type, I'm trying to create a state in which the "Assign
To" list changes dynamically according to the next state, e.g. When a user
selects the next state to be Active he'll get to choose from all
Development Leaders, and when he chooses Review he'll get to choose from
all Engineering team.
I've tried this:
<State value="New">
<FIELDS>
<FIELD refname="System.AssignedTo">
<WHEN field="System.State" value="Active">
<ALLOWEDVALUES expanditems="true">
<LISTITEM value="[project]\Development Leaders" />
</ALLOWEDVALUES>
</WHEN>
</FIELD>
</FIELDS>
</State>
but it doesn't work. No matter what I do I get a list of all the users in
the system.
Is this even possible to do with the "Assign To" field? If so, what do I
need to do to get it to work?