Connecting an domain

As the title states,

I’m working on a saas tool that requires the user to being able to linking their website. How do I do that?

To use the application I want the user to Sign their name, password and domain-name into a menu.

When the user has filled out the information, the user will have activated an account registered to their website.

Is there any API or “how to” for doing something like this? I’d appreciate any help.

Simply purchase a paid plan, then add the appropriate records via the settings page