Many individuals have created a version of this circuit before. So, I take no credit.
Most USB power banks have an auto shutoff feature that is triggered when the output current drops below a threshold value. I am guessing whatever value that is, will vary greatly from unit to unit so you may need to adjust the load resistors.
This circuit kept my dollar store power bank from going into auto shutdown.
The oscillation frequency was approximately 1/7 Hz.
So every 7 seconds a ~100mA load is applied for ~100 ms.
The advantage of a pulse load circuit like this is that the average load current can be kept low, instead of say a static 100 mA load on your power bank.
