Question

All Halloween lovers should know what to say when knocking on the door! “Trick or _____”

Answer 1

All Halloween lovers should know to say "trick or treat".

Answer 2

The answer of your question is "Trick or treat"
This is the special custom on the occasion  of Halloween for the the children.
Children at the evening wear the costume and goes from one house to another asking for trick or treat. People usually gives them treat generally in the form of candy or money etc.
The trick is work as the sign of some kind of idle threat for the property or something  of the people don't give treat.