It’s part of the lnurl-pay protocol . The receiver’s client has to store and handle the message on the lnurl server
Discussion
Didn’t know this one. Found it’s specified in LUD-12. Opened issue for SatSale to implement this. This makes it easy to allow Lightning address payments for, for example, paying invoices (where you need to specify invoice number along with payment). https://github.com/SatSale/SatSale/issues/134