Yes, thanks! So, it will still be hosted by Leadpages when you’re done? You don’t have some other hosting provider that is hosting http://longstripecreative.com/?
In this case, do they have somewhere in a control panel where you can upload a certificate for your custom domain? Is their link to Let’s Encrypt something like “Go to Let’s Encrypt, get a certificate, and then upload the certificate here once you have it”?