Hi
Good question. I'm actually just helping a startup with this.
Suggestions (some obvious, some less):
1. Forums (in your relevant service).
2. FB groups.
3. Partnerships with HR people in that field.
4. Post Job opening.
5. LinkedIn.
6. Contact them directly, and give them an incentive to post/share that they've joined your platform (which will bring some of their friends).
There's a lot more - depends on the field.
Good luck
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