Smart Campaign: Send Email based on form checkbox

Hello,

I have a Marketo form with a checkbox.

I created a Smart campaign with Smart List trigger: Fill out Form.

I added a Flow: Send Email.

I would like to only send the email if the checkbox is checked. I don't see anything in the trigger "Contraints". How would I go about triggering this Smart Campaign ONLY IF the form checkbox is submitted as checked (true)?

Thank you!