Custom URL on Invoice Layout

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Hi 

New here . Not new to pastel , been using for 16 years. 

I am creating a new layout for a new company. 

I want to add a Payfast pay now button to my invoices , quotations , proforma invoices / sales orders etc. so customers can pay via credit card. 

It may sound complicated however this is very simple. It is merely a url . 

It can be a static URL and the customer can enter the amount needed to pay manually before proceeding to payment or ideally it can be a dynamic URL generated by pastel that includes the invoice amount and the invoice number in the URL and all the customer has to do is click the link and pay. 

I searched everywhere in the forms designer and I cannot find a way to add a URL link to the layout 

I would like this link to be picture , the payfast logo but when clicked send to URL. 

Any ideas on how to do this , will be greatly appreciated.