Behaviour of "+" button

Hello

I created a subgrid on a custom entity to show all task records.
In the OnLoad event, I use fetchXML to change the list to all tasks of the child records and the tasks of the record itself.
This is working like a charm.

Now I have the problem, that the "+" button always opens an empty record in the same window with no regarding set so I have to look up the record I want to add the task to.

I want to set the regarding of the record when I press the "+" button automatically. I think this should be the "Add New {0}" button, but I cannot let it show via Ribbon Workbench.

Do you guys have an idea?

Thanks.

Best regards