I have a website that let's users create a subdomain app like this: https://subdomain.mydomain.com

I want them to be able to have the app appear as if it's on their site, like this: https://myapp.theirdomain.com or maybe https://theirdomain.com/myapp/

I'm using Heroku and was wondering if this is realistic and how the ssl certificate would be done such that it worked on both domains? If not, can I get it at least working on their domain?