connect subdomain to another domain

please can someone give me a clue and the right direction to go if I wish a user to be able to connect their own domain to jumpstartapp subdomain (assuming a user have already a working subdomain on my multitenancy app where DNS target is just a subdomain set in heroku)

in other words if I have subdomain.myjumpstart.app then I visiting this subdomain I should end up at mysite.com

I need a proper solution not a mere redirect.

I read that I need to set CNAME for subdomain.jumpstart.app with a value of mysite.com but this does not work in itself so I must be missing something. there was also some mentioning about A record of the main client's site (like mysite.com) pointing to multitenancy app. but I am not sure what is the A record of the multitenancy app is when it is hosted on heroku.

 another question - what should I do if I want a user of subdomain.myjumpstart.app to send emails where from and reply to addresses will show mysite.com domain and not the myjumpstart.app domain
what do I need to do in this respect using a typical mail provider like sendgrid

many thanks in advance, any advice will be greatly appreciated