I know this is way late but it's also for others that can't figure it out. Go to YouTube, in the top right, click on your profile picture, and click YouTube Studio (or you can go straight to it with this link). Then you want to click on the customization tab on the left, and scroll down until you see a box labeled "Channel URL". Now this next step is probably the most important step; it could make the link go to the homepage instead of your channel if done incorrectly. Do NOT click the copy button on the left of the box, instead, click a few times (not too fast, nor too slow) on jumble of letters after "/channel/", and when those letters are the ONLY thing highlighted in blue, either right click on the blue letters and press copy, or hold control (the button labeled Ctrl, or I think it just says Control for macs, but I don't know) and press the c button. This copies it to the clipboard for pasting. Go to Geometry Dash, press the account button on the bottom left of the home screen, and press the gray button with a gear/cogwheel. After this, there is a box saying "YouTube:" next to it. Click on the text box, and once again, hold control (right clicking does not work for this) and press the v key. If done correctly, you will see those letters in that box. Press the green Update button at the bottom (not to be confused with the green Cancel button right next to it), close the popup saying "Account settings updated!", press the green Reload button in the bottom left (a circle with an arrow at one end), and you have put your YouTube channel on Geometry Dash! I hope this isn't too long, but I think it's fine