I want to open another site in newtab under my site iframe.Please help!

Hey hi all,
this is my subdomain : https://themes.blogspiceup.com/

and i want iframe like this website : https://www.btemplates4u.com/

whenever i click on “demo” of this website it opens demo of that theme or template in its frame.

Please help regarding this.I have search Google and everywhere but i didn’t get complete tutorial to make this thing work.