How does the user recruitment process typically look like for user research & usability tests?

When you are working with an actual client to enhance the ux of their product, at the research stage of the project how do you recruit the users?

In other words, let's say you start using a tool like Optimal Workshop to run a study. Do you send the link of the study to the client and they forward it to their users or does the client provide you with a list of contacts of which you send the link of the study directly?