Copy UDF from PO Line to AP Invoice when entering AP Bill from PO

Could someone please help in providing a solution of moving the UDF from PO into AP Invoice?

I am new to developing in acumatica. I have created the user defined fields in POLine and APTran.

While reviewing the source of APInvoiceEntry, I looked at overriding the method CopyCustomizationFieldsToAPTran.

Code I added is below.

public void CopyCustomizationFieldsToAPTran(APTran apTranToFill, IAPTranSource poSourceLine, Boolean areCurrenciesSame, CopyCustomizationFieldsToAPTranDelegate baseMethod)
    {
  
      baseMethod(apTranToFill,poSourceLine,areCurrenciesSame);

      var portID = poSourceLine.GetExtension<POLineExt>()?.UsrPortID;
      var tranline = apTranToFill?.GetExtension<APTranExt>()?.UsrPortID;         
      
      if (tranline != null && portID != null ) tranline.UsrPortID = portID;

    }

I am getting validation errors when I try to publish.

error CS1929: 'IAPTranSource' does not contain a definition for 'GetExtension' and the best extension method overload 'PXCacheEx.GetExtension<POLineExt>(IBqlTable)' requires a receiver of type 'IBqlTable'

error CS1061: 'string' does not contain a definition for 'UsrPortID' and no accessible extension method 'UsrPortID' accepting a first argument of type 'string' could be found (are you missing a using directive or an assembly reference?)

error CS1929: 'IAPTranSource' does not contain a definition for 'GetExtension' and the best extension method overload 'PXCacheEx.GetExtension<POLineExt>(IBqlTable)' requires a receiver of type 'IBqlTable'

Am I using the correct method to accomplish the task?