Sub grid - how to avoid the search option while creating new record

Hello 

I created a new entity , which link to additional entity and shown as a sub grid. 

I would like the users to be able to create a new record from the sub grid using the + sign, but without giving them the option to search an existing record. Once they push on the + sign, a new form is open . 

I saw this on other system entities (like new opportunity on contact - a quick create form is open) but can't find the way to set it on my sub grid  

Thanks