What if you're asking a vendor to add their payment details but they're already a Melio user? When they click on the link to fill their details, they'll have the option of selecting their bank account on Melio as their receiving method (like in the screenshot below).
If you're the vendor: When requested to enter your payment delivery details, just click "log in" on the top of the link you received. Then just log in to Melio to see your bank account as a payment delivery option (like in the screenshot below).
As always, if you have any questions, reach out to our support team at firstname.lastname@example.org.