Hello, @taylorsfittingan.
When you sign up for the trial subscription, you're asked to enter your email address and password. You can use the same login credentials when setting up your payroll. Here's a short video clip on how to set up your payroll in QuickBooks:
You can check these articles for more insights about running payroll in QuickBooks Online:
Keep me posted if you have other questions. I'm always here to help.