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Is it possible to make a seamless loop (sort of like a button loop) at both the beginning and the end of the bracelet? I'm actually making a guitar strap and a tutorial would make my life WAY easier!

Thanks!

Yes, that is possible.
A buttonhole at the start is shown in this tutorial
If you want same endings on both start and end I would suggest to make them both after finishing the knotted part of the strap.
So start knotting with some loose string on top, instead of starting the V-shape like in the tutorial. And end both ends in the V-shape.

We do not have a full tutorial for ending like that, but it is a bit similar to the loop-and-braid method, where strings are folded over to end them in the loop: loop-and-braid.
If you do the buttonhole (and not a loop) you can use the same technique of strings folded over on the edges to finish the string-ends.