Create form and Mandatory relation

I created a new entity with N:1 relation to the account , and added it as a subgrid to the Account form. 

If I remove the Mandatory from the Account field on the new entity, the + sign on the subgrid behavior changed . it open a lookup instead of a new create form . 

I understand why it is like that, but wonder if there is a way to bypass it somehow. I want it to not be mandatory and open the create form (even with a code change) 

Thanks, 

Tali