Hi
This is a classic market place which requires marketing to both sides simultaneously.
I would start by selecting the most popular categories/fields that the users (employers/companies) are looking for (based on Google Adwords Keyword Tool or maybe checking Fiverr - if their services are the similar to the ones you plan to offer).
Then, do intensive, but focused/targeting marketing* for freelancers, and the employers/companies in the above fields that you chose.
Repeat the above process for the top 5 fields/categories and then either repeat or move on to the next categories. The challenge with marketplaces is that you need to have a decent amount of freelancers and employers at the same time – and therefore it is better to focus on a limited amount of fields/services in the beginning and then slowly expand.
As for the actual marketing methods, this really depends on the types of freelance services that you’re offering and your budget, but the classics are LinkedIn, Facebook, Indeed and specific forums.
I’m actually currently consulting someone on launching a market place and would be happy to help if you need further advice.
I've successfully helped over 350 entrepreneurs, startups and businesses. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david

Demand exists. Simple!

Usually on the first stage they collect the project all over internet: other marketplaces, social networks and so on. Then, after they have 50-100 tasks they turn on adv in search and social. Guys who want to earn money 10 times more than the guys who want to spend it:)