When you create a new custom form, it is available to all sessions you create after you created the form. It doesn't automatically populate as an option for any sessions created before you created the form. However, you can get it to populate, and here's how you do that:
If you aren't the facilitator of the session, you'll either need to work with the facilitator to edit the session, or you can add yourself as a facilitator, so you can edit the session.
Once you have the ability (or the facilitator does) to edit the session, in the upper right-hand corner, click the pencil button, and then select "Edit Session."
Update the center to be unassigned. Wait a second.
Then update the center back to your center.
You should now have the new form available in that session.
This only needs to be done for sessions created before the form was. All new sessions will have that form readily available.