Help with Getting Data from Form/Report

Help with Getting Data from Form/Report

I have the following in a POST action that works just fine ..

<input type="hidden" name="criteria" value="ID=2907418000008392079">

.. and returns the following record/json response ..

{"Form_Users":[{"Name":"Smith, John","Customers":"[Cafe]","Email":" me@me.com","ID":"2907418000008392079","Customers.Cust_ExtId":"[1187]"}]}

My question is, how do I format my criteria parameter to return the record where Cust_ExtId = 1187 ?

These are not working :

<input type="hidden" name="criteria" value="Customers.Cust_ExtId=1187">

nor

<input type="hidden" name="criteria" value="Cust_ExtId=1187">