How can one get freelancers registered on my freelancer website?

You are going to need to build a funnel so you can convert cold traffic using paid clicks.

I would start with freelance registration so you can get a base of people.

Once you have that base, you can start attempting to get paid clicks by business owners, or maybe a PR blast that gets some traffic your way from businesses.

Marketplaces are not easy, because it is a chicken and egg situation, that usually requires a ton of cash to get the wheels turning.

The reason why only a few survive, and others die or merge, is because of this reason.

If I were you, I would go super niche. Think awfully specific with your freelancers.

Designers for specific businesses or tasks

Writers for specific purposes.

RPA Developers that only concentrate in a certain area of a project.

Super-duper expensive pros.

Super-duper cheap help.

Think Asiarpa.com. Think RPA Freelancer

If you try and do it all, like Upwork, you are going to get smoked IMO.

Hope that helps!