You don't need to limit yourself to just Upwork; your target audience consists of freelancers. If your product strategically addresses a freelancer's pain point, they will be interested in making a purchase. Use platforms like LinkedIn or Twitter to your advantage.
Engage with users on Upwork's Twitter profile, and soon you'll notice an increase in Upwork-related content on your feed, alongside your own content receiving attention.
Foster these connections by regularly posting tweets that resonate with your ideal customer profile and address their pain points - just like many successful SaaS founders have done.
As a bonus tip, consider hosting info product giveaways that are relevant to your niche. Share these giveaways through platforms like Notion and pitch your service alongside them, making sure that your SaaS product is an essential part of making the most of the giveaway.
Cheers!