From my experience it is best to assemble a sales team and make them do some cold calling. I believe that speaking to start-ups personally will gain you more companies to post jobs on your platform.

I would also advise you to go on LinkdIn and look around the job listings there to see which companies are offering them; then contact them to perhaps post on your platform.
You should show them why you are better and why it will benefit them from posting on your platform versus others. I believe that finding the companies through LinkedIn and such will make you able to attain a large customer base rather quickly.

If you need anymore help feel free to give me a call. I am currently doing a free 15 minute consultation so just message me for the discount code.