What about ssl certs for custom domains? I built a simple web app generator (forms, listings, etc.) and I had a clear roadmap on how to add custom domains but

What about ssl certs for custom domains? I built a simple web app generator (forms, listings, etc.) and I had a clear roadmap on how to add custom domains but got stumped in handling ssl. It was a toy app for me so I kinda stopped working on it so never really put too much thought on how to handle that problem.