but I need a proof that this payment belongs to my user account !!
Usually, these kind of sites generate a unique deposit address for each user. That's the way these get tracked. The message in the uri you posted is for your information only, it never ends up in a block.
This is correct. Don't worry in proving that this payment came from your account.
However, it they still want a proof you can sign a message just like Theokool1s said.
Just transfer and don't worry. This address was generated exclusively for your purchase/transfer. No other user received that address as well (it would be highly unlikely)