How to make a widget with an invitation to register?

Question

Adaptive design

I ask the question, because I don’t really understand how to set a task for a freelancer. But maybe I can do it myself if I figure out how.
It is necessary to make a widget appear on the site that will offer users to register and receive a certain amount to their account for this. Something like an invitation to a chat, but instead of a chat - registration. Correct display on PC and mobile is important.
Custom CMS, the site has 2 languages, each language needs its own widget.
How can this be implemented? Are there any ready made solutions? Is this somehow possible to implement through javascript, or is it something else?