All Stacker portals get a free something.airportal.app URL based on the name you gave for your portal when you created it.
How to change your portal URL
From Setup Home choose App Settings and then expand Change your URL.
Here you can set a new URL by entering it in the box and pressing Save.
Changing your portal URL will break your previous portal URL, so you will have to inform your users about the change if you have already invited them to the portal. Their existing emails and passwords will continue to work.
Changing your portal URL will allow someone else to use the URL you used to use. Nothing to be concerned about necessarily... just something to be aware of.
Stacker can work with your custom domain on our Pro plan. Find out more.