Unregistered 4U, you were right.
Well, after the suggestion of jlc1978, I tried charging with mbp powered off.
The good news: the 'experiment' succeed!
But, the other news, were that it was really slow.
I did not expected the speed of magsafe, but it is ok.
You know, I wanted this to succeed, just to have it as an option,
only in -let's say- emergency state.
I am not good in all these electrical terms,
but it is rather strange to to me,
that I left mbp before closing down with a battery of 99%,
and the powerbank after 1,5 hour has gone from it's 100% power to 94%,
and it was still charging,
but I interrupted the procedure then,
as I wanted to use the mbp.
I think mbp has about 8000mah batteries in a whole,
while powerbank has 20000mah batteries.
So besided the slowness,
I cannot undestand, how it is going,
taking about 6-10%(approximatelly, as I interrupted when the powerbank was down to 94% of its charge) from the powerbank's power,
to just give 1% in mbp power/battery,
when the one has 20000mah battery and the other 8000mah battery.
I guess that the whole 100% of the 20000mah powerbank, could just transfer about 10% of power in mbp,
before it completely runs out of power.
In any case,
now I now that usb-c charge, at least it works.