I would suggest overriding the Release action and performing the field update during the override.

This answer may assist in writing the Release override: https://community.acumatica.com/develop-customizations-288/how-to-override-an-action-in-a-graph-extension-11328

@darylbowman Thank you for the link. 

I read through that and, frankly, I am not following how that works and where and how in the code logic I would update the UDF.  (I am still learning the Acumatica C# code functions)

Also, in our instance, it appears there is another customization that already performs a release override; would I create a conflict?

Finally, in the example given it refers to SOOrder, would that class also pickup both the AR301000 and SO303000 screen functions for creating an invoice?  (again my apologies if this is a stupid or naive question.)
Jeff

Ordinarily, I think this would do it, and you’d want it in a graph extension of ARInvoiceEntry:

public delegate IEnumerable ReleaseDelegate(PXAdapter adapter);
public IEnumerable Release(PXAdapter adapter, ReleaseDelegate baseMethod)
{
	var result = baseMethod(adapter);

	ARRegister register = Base.Document.Current;
	Customer customer = Customer.PK.Find(Base, register.CustomerID);
	Base.Document.SetValueExt<ARRegisterExt.usrInvParent>(register, customer.ParentBAccountID);

	return result;
}

I’m not honestly sure how it works to have multiple overridden actions.

@darylbowman Thank you!

I will give that a go later this evening and post back here if I have an issue.

@darylbowman 
This is where my knowledge regarding Acumatica completely breaks down.  There are so many graph extensions with ARInvoiceEntry  in them, I simply chose the first in the list.  

However, during validation/compile I have the error below.

Building directory '\WebSiteValidationDomain\App_RuntimeCode\'.
\App_RuntimeCode\ARInvoiceEntry.cs(58): error CS1503: Argument 1: cannot convert from 'PX.Objects.AR.ARRegister' to 'PX.Objects.AR.ARInvoice'
\App_RuntimeCode\ARInvoiceEntry.cs(58): error CS1503: Argument 1: cannot convert from 'PX.Objects.AR.ARRegister' to 'PX.Objects.AR.ARInvoice'

Here is the code in the extension below.  What am I doing wrong here? (for future reference, how do I know I am choosing the right graph to extend?

-------------------------------------

namespace PX.Objects.AR
{
  public class ARInvoiceEntry_Extension : PXGraphExtension<PX.Objects.AR.ARInvoiceEntry>
  {
    #region Event Handlers
    public delegate IEnumerable ReleaseDelegate(PXAdapter adapter);
    public IEnumerable Release(PXAdapter adapter, ReleaseDelegate baseMethod)
    {
      var result = baseMethod(adapter);
    
      ARRegister register = Base.Document.Current;
      Customer customer = Customer.PK.Find(Base, register.CustomerID);
      Base.Document.SetValueExt<ARRegisterExt.usrInvParent>(register, customer.ParentBAccountID);
    
      return result;
    }
    #endregion
  }
}

You did that right. This was my fault. Replace ARRegister with ARInvoice everywhere except ARRegisterExt.

Hey @jeffgrammer69 

I just realized I missed the ePXOverride] attribute above the method. That may present issues.

It should be:

namespace PX.Objects.AR
{
  public class ARInvoiceEntry_Extension : PXGraphExtension<PX.Objects.AR.ARInvoiceEntry>
  {
    #region Event Handlers
    public delegate IEnumerable ReleaseDelegate(PXAdapter adapter);

     PXOverride]
    public IEnumerable Release(PXAdapter adapter, ReleaseDelegate baseMethod)
    {
      var result = baseMethod(adapter);

      ARInvoice invoice = Base.Document.Current;
      Customer customer = Customer.PK.Find(Base, invoice.CustomerID);
      Base.Document.SetValueExt<ARRegisterExt.usrInvParent>(invoice, customer.ParentBAccountID);

      return result;
    }
    #endregion
  }
}

Good evening Daryl!

My apologies for taking so long to respond.  I was traveling on Friday.

The PXOverride] missing was exactly what was causing me some issues and I did not realize I needed that command in there.

This works PERFECTLY!  And works from either the SO302000 or AR301000 screens.  

Thank you!