Help using iframe

I’ve seen and read my posts concerning this topic, nevertheless I haven’t found any concrete answer as to how I can achieve this. I would like to use an iFrame to do what was done with the following app:

I want to use a personalized domain like that website. Any ideas on how I can get this done?
Also in that example, are they paying for the domain “” or is there a way to get that done without paying for a domain?
Finally, can this be done with an app that requires signing in?

Thank you