As QuickBooks Online does not support Sales reps, you can create a custom field in QuickBooks to be able to sync them with InSitu Sales.
To create custom fields in QuickBooks Online you must click on the wheel-looking icon on the top right.
Then, you will click on Account and Settings.
On this screen, you will click on SALES.
Here you will see CUSTOM FIELDS in bold, click on the field.
Once you click on the text this window will drop down.
On this screen, you will add the name of the custom field you need.
Once you are done click on SAVE.
To set up the custom fields in inSitu, you must go to the integration screen by clicking on Integration on the side menu and then on QuickBooks. Select Sales Rep from the drop-down under Custom fields.