In order to assign a partner carrier to a driver, first go to Menu > Fleet > Drivers.

Next, open the required driver’s profile and under “Payable To”, select the partner carrier you wish to assign to the driver.

Note: If you only want to create a settlement for the partner carrier (by vehicle) you can turn on the toggle “Disable Settlement”. However, if you want to create settlements for the driver as well as the partner carrier, then keep the toggle off.

Scroll to the bottom and click the 'Save' button.