(Might be useful to others.)
I did not want the path of the onboard LiPo battery (difficulties to source a good one). I planned to use one of my USB powerbanks instead.
And then I realized that the Morserino doesn't draw enough current. After a while the powerbank goes to sleep (assuming no load).
But I solved this by using this little device from Sotabeams: "USB battery pack keep-alive load" (https://www.sotabeams.co.uk/dc-power-radio-shack-or-portable/). I mounted it, and now have the perfect combination.