How do I allow my freelancers to send me a payment request?

Under "Workforce" go to your freelancers and start a "Direct collaboration" with them
This allows freelancer to send payment requests to you. 

You can give that "Direct Collaboration" session a title and define the payment method (hourly vs. deliverable). If necessary, enter the agreed hourly rate and specify whether the expert is allowed to adjust it for future payment requests.

Freelancers you have hired via a job posting can automatically send you payment requests for their bookings.