In our Dynamics CRM online custom project - we've the default ACTIVITIES tab in a custom entity named DocProject's form

ACTIVITIES is able to take multiple entries.

Okey, this is done by Dynamics CRM guys. So far so good.

In the same form, we also have a DocProjectActivities lookup field for a custom entity DocProjectActivities

This is a lookup field, hence it has got a 1:N relationship.

Problem:

How do one makes sure that this custom entity DocProjectActivities allows to make multiple entries as that for ACTIVITIES?