Private panel
For some projects, you could be looking for IT decision makers at medium- to large-sized companies. For example, if you wanted to sell rack-mountable switches, even if you craft a screener for this, chances are that it will not find any users. This means you need a private panel. For other projects, you want to run the test with your actual users--for example, testing an employee-only website or the user account section of a public website.
This needs some extra work on your part, but most remote testing platforms will help you. Basically, you need a list of e-mail addresses from people who agreed to participate in the test. Then, the magic is done by the platform, to make sure that they can do it properly.
Note
Remember, just because some of your users agreed to take the test, by clicking a link in an e-mail, for example, they might not do it. If they try, they might have technical difficulties, low-quality recording with inaudible voice, or other issues. Some will do the test...