Break a chopstick or something similar and hot glue it in place. Make sure your spool moves freely on it first though.
Another option that I’ve used before is just getting a cone of thread and setting it on the right side of the machine and I kinda zig zag between the bobbin winder and the thing that stops the bobbin when its winding (I should know these names, I sew for a living. lol) and then pull the thread over to thread it normally. It can get a little more finicky when sewing though sometimes so you have to be vigilant as you sew, which I know isn’t best for a beginner. But it does work!
I set my machine up next to my serger, and I sew so much that I have to use cones of thread, so I set the cone of thread of the end of my line of serger cone station things, and they have the little hook thing above the cones and I just have the thread go off that and then thread my regular machine normally. But when I go to conventions and bring my machine I don’t bring my serger so I had to figure out that little cone of thread sitting next to the machine trick.
If all else fails, you can absolutely find a replacement part for that. Its probably one of the most common parts to replace and it will most likely only be a few dollars. Check at craft stores around you for one, or see if there’s a Brother dealer in your area. Or order one online.