Can I add reference after payment？
If you’re sending a payment you have to add the reference while making the payment because it is sent as part of the payment.
I forgot once so I sent them 1p with the reference to help them tie it up 
Hi @Liu 
Welcome to the forums! Unfortunately you can’t edit payment details (like a reference) once it has been sent. However, if the other party needs the payment details, you can get in touch with us via the app for this. 
You can add a reference to the payment, before sending it. If you leave it blank, the standard ‘Sent from Monzo’ reference may appear.