Hi @ViweM  We need to have small customization to do this. Here is the code for reference.

Also. attached screenshot and customization package for reference. You can deploy this package to your instance and verify.

Customization: Line level Customer ID has defaulted to the Header Customer ID, and if required we can the Line level Customer ID

DAC field

 public class ARTranExt : PXCacheExtension<ARTran>
    {
         PXUIField(DisplayName = "Customer ID")]
         PXDefault(typeof(ARInvoice.customerID))]
         CustomerActive]
        public int? UsrCustomerID { get; set; }
        public abstract class usrCustomerID : PX.Data.BQL.BqlInt.Field<usrCustomerID> { }

    }

The customization package worked perfectly, thanks a @Naveen

Most welcome @ViweM 

Hi Naveen

I seem to have an issue, I tried adding a new custom column on the screen on the same customization package but I’m getting a conflict error. I even tried to add it through a separate customization package but I get the same error, I’m adding the new through the front end.

this is the error : 

Conflict control ID:phG_tab_Items#0_grid_Levels#0 from page ~/pages/ar/ar301000.aspx

I actually changed levels of customization projects and it worked no need to worry

Yeah. Thanks for the update. :)