Saylor posted to twitter about this
I tried it and it works even though it's a little buried, and it's not a button.
  • in cashapp, click on the "home" icon in the lower left
  • click on the bitcoin chart below "add cash" below the three blue buttons, click the text "recieve bitcoin"
  • click the big ugly 2d barcode
  • click the copy icon to the right of "lighting network" to paste the lnurl to your clipboard
This is a lnurl you can paste it into another lightning wallet (strike, muun, bluewallet, WoS) and specify the amount to send from the outgoing wallet. The UI won't update when you send the funds, but you will get a notification from cash app in your notification tray. Then when you refresh your screen, the bitcoin balance will show as dollars at the top of the screen, but if you scroll down you will see the sats balance.