Income based on a user-based model

Hi!
I’ve just started as a sole proprietor in my secondary occupation.

I’ve secured my first assignment, where I’ll essentially be paid based on a user-based model via an external platform. So the more my services are purchased on that platform, the more I’ll earn.

How should I invoice this? Do I send a monthly/yearly invoice to that external platform as soon as I know what I’ve earned that month?

Thanks!