How we can integrate Hugo website builder to our registrar where we sell the domain
Hugo has nothing to do with your “domain”. The “domain” is just a title for your web property. The property itself is something (a website) that is accessible under that domain. So the proper way to “sell” your Hugo website with your domain would be some form of contract where in connection with your domain name you hand over ownership to a Github repository (or any other repository) that has the history and current state of the website in it. This way the new domain name owner can continue the development from where you were at the point of sale.
This requires someone on the buyers side who understands how Hugo is used to create a website. There is no “idiot proof” way of setting up the website for the buyer to not know anything about the things that are going on between the domain name and the site itself.
We have domain-selling websites like Namecheap and we want to integrate website builders like hugo
So I am asking the process for it
No idea what you mean with “web site builder”. Installing Hugo binary with your product should suffice if your customers want to use a static site generator.
Hugo is not a drag and drop builder. You just can’t have it installed e.g. with Softaculous.