Feature Request: Reference Text

Like in your simpler plugin “Easy Accept Payments for PayPal” it would be cool if the customer could enter a reference text at checkout for certain products. It would be optimal if it could even be checked in advance whether an entry has been made. So that the reference text is a mandatory field.
For my use case, I can not use the plugin as it is currently. We sell a digital certificate, which requires that the customer enters his domain in a free text field. Maybe you can further develop your plugin here.
Many greetings
Oliver