Questions Making Sales Person Field Mandatory on Customer Master

Hi everyone, 

I'm trying to make the Sales Person field mandatory when creating a new customer. 

I've used the help file, which was useful in assisting me in making Phone and Email fields mandatory.

https://help-2020r2.acumatica.com/(W(4))/Help?ScreenId=ShowWiki&pageid=97f5b92f-906a-4edd-ae38-f5605e83d845

However, within the customization, under edit attributes for the AR.custSalesPeople.SalesPersonID, there isn't field an customise attribute 'PXUIField' available. This attribute is available for Email and Phone fields. 

Is there another way I can achieve this? 

below is the PXUIfield available for Email

 below is the customise attributes for Sales Person, PXUIField not available.

thank you for your time and input in advance!

Adam