PayPal Gateway Setup - Link in Email Invoice

I have installed the PayPal gateway app and am needing some help.
I want to include a “Pay now via PayPal” link in the invoice that gets emailed to a customer after creating a sales invoice. In apps like Xero the “Pay now via PayPal” link takes the customer to the PayPal site and asks them for the correct invoiced amount.
Has anyone built anything like this?
Any help would be great

You have to create a Payment Request once you have your PayPal app setup.

can you explain payment request and how to add a link to the invoice?