You can make 500 bitrate look like 1500 by performing a miracle. Or with other words: you can't. If your upload allows only so much data, you cannot magically squeeze more data through it.
However, you can make the best use of the available bitrate, whatever limited it is. Reduce the output resolution. Find the best ratio between resolution and visual quality. Find a resolution as low as possible where the image isn't dominated by compression artifacts, but still as much detail visible as possible. Half the resolution means quarter the bitrate requirement.
Use lower frame rates, not higher than 30, better 25 or even 20 or 15. Half the frame rate means half the bitrate requirement. Rather flicker from low fps than a bloody pixel mess.