To be clear, once you export the file plays correctly in your media player, but gets darker after uploading to YouTube? The rest of this post assumes the answer is "Yes."
If so, well. YouTube recompresses all uploads to its own format you have no control over this final stage, and it's not a Hitfilm problem. You could try brightening your video in Hitfilm to compensate, but it's YouTube processing that would be causing the problem.
Unfortunately not, because you don't have any control over the YouTube encode.
A while back, I had to do a number of videos for multiple platforms. I made export luts in premiere that I used for each platform (youtube/vimeo, etc).
To mirror what Triem23 said, video hosting platforms will encode the file for playback and this can bring about changes to the content. Definitely look into making a preset to adjust your output for the platform of your choice.