Most "powerbanks" are designed to cut internal operations off when the current falls below certain threshold (50 mA - 100 mA). Having additional load to keep it running will waste the battery runtime.
The best way is to modify the powerbank circuit to remove this protection. Some cheap powerbanks have a single control circuit that apparently has the current shunt circuit built-in, and it is not possible to disable. Some powerbanks will have low-Ohm sensing resistors on-board, so increasing their values will lower the cut-off threshold, but it may have adverse effect on the overall current delivery. It might be possible to somehow fool the cut-off circuit applying some internal bias, or else, it depends on powerbank control circuitry.
FOR ILLUSTRATION: My Powerbank (Model CJ-6000) has a pretty sophisticated circuitry, I might start playing with it:
See also https://electronics.stackexchange.com/a/270342/117785
ADDENDUM: after some reflection (based on powerbank circuit shown above) it becomes clear that a simple change in current sensing resistor will not work well - while the cut-off threshold gets lower, the overcurrent will also go lower, which might be unacceptable. The workaroud would be to use a non-linear element like a Schottky diode, with something like 10 Ohms in parallel (to define new cut-off threshold). This circuit has something like 3.5A overcurrent threshold (which amounts to 350 mV threshold over 100 mOhm shunt), so the diode must have a pretty low forward voltage like this one to avoid the overcurrent flip.