Queries :: Retrieve Criteria From Form / When Form Is Closed

I have a query that retrieves one of its criteria from a form. I have referenced correctly the value on the form from the query, and it works, but what I wanted to do is a bit more complicated: when the form is closed, I want to launch the query with a "default value". I tried to do it in 2 different ways:

a) Defining an "IIf" at the query criteria: I would need a function that checks if the form from which I retrieve the values is open.
b) Defining public variables with a default value, which would be changed from the form: I don't know where/when to initialize the value of the variable.

Query gets criteria from form when it's open. If form is closed, query uses default value.