product.type fill button (links)

Hi experts,
I need your help please. (HPSM 9.30)

We have created an assignment field in the producttype table. We have added the assignment groups for each record in the product type. Now whenever we are creating an interaction, once the service desk select the correct categorization, automatically the Assignment group is populated in the Interaction form.

However there are some times where the assignment group should not be the predefined one. 
So if the service desk have already selected an assignment group which is different from the one defined in the  the categorization, the assignment from the categorization overrides the one selected by the service desk agent.

What I need is the following:
- If the Service Desk agent selects the assignment group before categorizing the ticket, & when he categorizes it, then the Assignment group should remain the same (even if the assignment group selected is different from the categorization)

- if the Service Desk agent categorizes the ticket before selecting the assignment group, then when they click on the fill button of the product.type field, then the assignment should be populated automatically.

Can you please let me know what should be the query in the incidents links?

Thanks