In Microsoft Dynamics CRM, to add a record to a subgrid on a form, you hit the + button in the upper right hand corner. You will notice that for some subgrids you will get a lookup field, while others will give you a new record form.
The reason for this different behavior is that the new button (+) can either function as “add new” or “add existing.” For example, when you are adding an opportunity, you probably will want to have the new button create a new opportunity, while when adding a contact to an account, you may want to have the user select from an existing list of contacts (or search existing first, then add a new one).
To control the behavior of the new record button on subgrids, look at the child entity being selected in the subgrid..If the lookup field for the parent is required, the user will get a “new record” form when clicking the + button. If the lookup field for the parent entity is not required on the child entity, the user will get the lookup field to “add existing.”