subgrid +-button loading "new" Form in same Page than open a new Window

Hi,

we are using CRM 2013 onpremise (no UR installed).

On our opportunity-form we have a subgrid to the opportunity-products.

When i click on the '+'-button, the "new form" of the opportunity-product loads on the same page. I exptected a popup-window like the other subgrid (to quotes).

can someone tell me how I can change the behaviour to open a new window?

I already added a lookup on the opportunity-product-entity with "business required" but without any success.

thanks a lot.

Tobias