By default, applications are deployed under the serverpilot user. If needed, you can create additional system users to run specific web applications.
For the relevant server, visit the Users tab and click + Add System User.
Enter a name for the user and click Create.
Return to the Users tab and you should see the new user you just added.
Now, when you visit the Create an App form and select the relevant server, you will be able to deploy the application under the new system user.
You can find more information on the What are System Users? page.