How can I attract a large number of service providers before launching my on-demand website?

If you're in marketing, a great way to attract people to your website is through blogs writing. There are so many topics you can write about. Teach people something and they will come vack for more. The important thing when writing a blog is not selling your product. You have to write something usefull for your potential users to create awareness. Slowly lead them further down your tunnel.

At the end of your blog you could send them to a landing page related to your blog post. After a few blogs and after you have enough people reading your blogs, you have to write one blog talking about your product/service and refer them to a landing page that explains more about what you have to offer.

If you have any questions. Call me. I can tell you more about what's important for your blog posts. What to inclide and what not to and help you gain loyal potential customers for your platform.

Sometimes, new-kids also float wrong data to create hype and get initial users. So, I will take what others are saying with a grain of salt. What you should - as you are - focus on getting initial set of users. There are a lot of ways you can go about this:
1) Getting published in a leading online magazine a day before launch
2) Running highly targeted awareness focused FB & Insta ads a week before launching and then switching to use focused ads.
3) Display ads can also give a big reach
If you could share your Insta or FB page, I could share more. Let's schedule a call for more insights. For long term brand building, read this blog, https://www.upreports.com/blog/build-brand-identity-online-100-tips-ideas/

It sounds like you're building a marketplace, where you need to attract both buyers AND sellers. This is difficult because on day one you have absolutely nothing to sell and therefore no one to buy. My suspicion is that these other sites you're looking at are seeding their "service providers," to use your term. We had to do something similar when starting a job board. We had no job seekers (product) and no job posters (buyers), so we had to "fake it 'til we made it" by pulling in job postings from external sites to attract job seekers, then the job posters started to come, then more seekers, more jobs, and so on. But you do need to add value and not deceive anyone. If you help them, they'll appreciate your service.

Consider seeding your site with "service providers" or whatever you're selling. Lots of sites got started this way. I believe Quora seeded their site with both questions and answers for a long time.