Readsoft Invoices COM-plugin - How to change an Invoice Field Value

Question

I want to change the value of an invoice field of the current invoice being processed by the interpreter.

objInvoice = objEHApplication.CurrentInvoice
Dim aInvField As InvoiceField
aInvField = objInvoice.Fields.Item("theFieldName")
aInvField.Value = "someValue"   '// <- ReadOnly

The problem is that when trying to set the value property, the compiler says it's ReadOnly.

What is the correct way to change/set an invoice field value from an Eyes&Hands COM-plugin?

Installed version of Invoices: 5-6 R1 on win 7