Allow to create a "reverse" payment, so:
  • Select your own account to
    receive
    on
  • Enter the currency and amount
  • Optionally: enter an on ledger memo and/or private instruction
...
Share a unique link (using the OS share panel) to any app, eg. WhatsApp, Mail, Telegram, ...
Now if the recipient receives and opens the link and have XUMM installed, they can resolve the Sign Request just like any other request coming from a 3rd party app/platform/...