The FullName you set in your DataExt should be the same as the FullName for the customer you're creating. Otherwise, you'll be telling QuickBooks to set the custom field for some other random object.
e.g. if this is the FullName of your customer:
Dim str_name As String = "Louis Pluto Test 2 - ZA Code"
Then you should use that same FullName in your DataExt:
' This is *wrong*:
MyDataExtMod.ORListTxn.ListDataExt.ListObjRef.FullName.SetValue("Myname mylastname")
' This is correct:
MyDataExtMod.ORListTxn.ListDataExt.ListObjRef.FullName.SetValue(str_name)