Fetch Data from a form and input into a dropdown menu

Fetch Data from a form and input into a dropdown menu

I created a lookup field in form B called select event based on form A. After choosing the event, I want the registered names on form A to be listed in a dropdown menu or multi-select (Even better. For attendance taking purpose), is it possible? 

I tried using another lookup field for the names but it seems to give all the names for all events, I only want to the names from a specific event.