Another Lookup field criteria question

Another Lookup field criteria question

Need some help here. I have 2 forms called Action and Request. The Action form has 2 fields Tool_num and Category. The Tool_num is also common to the Request form. I want to add a lookup field to the Request form that will return all of the Category data that also matches the same number entered into the Tool_num field. The user can then picks the appropiate one from the list.
 
I tried to enter the criteria Category == Input.Tool.num . I have now read through a few forum pages and realise that it is not possible to do this. I have tried to populate the field values dynamically using some of the examples given in the forum but can't get it to work.
 
I am new to Zoho Creator!
 
Cheers, Adrian