Add MYOB Online Payment link to Invoice form

Hi,

I want to add a link in our invoice form that goes to the MYOB Online Payment System. when sometimes we print the PDF and send clients just the PDF (useful when sending from our own email domain). 

This: https://help.myob.com/wiki/display/ar/Adding+a+%27How+to+pay%27+section+to+invoices. is not very helpful, as if are sending it from our own email clients/domain we don't have any link like that. I couldn't see any field to add in the form that seem to point to something like the URL link to pay.

Regards

Damien