You'll want to log in to your Paychex account and go into Company Settings.
Select Integrated apps.
Click Create App.
Enter Application Name and Description.
Select Read & Write for both Payroll and Company & People. Accept the Legal Notice and Paychex Inc, API License Agreement, then hit Save.
You'll then see the API key and Client Secret, which you'll paste into Merge Link!
Paste your API Key and API Secret in Merge Link