The point of a loop would be that it can, well, loop around. The belt never stops. You can't link the ends into a seamless loop. Better buy a looped belt of approximate size and add something to adjust your construct to match the loop size. Leading the belt around 2 wheels in a // shape can help you tighten the loop.
But if you don't really need an endless loop, if you have something that is connected to the "loop" at a fixed point, like in your picture, then that is a different question. In both your picture and the one in the comments the belt isn't looped. Instead one end is simply folded over itself so the teeth bite and prevent it from slipping.
An alternative to this I like is to take both ends of the belt and turn them by 90° with the tooth facing inwards. Place a worm gear between those two ends and a hollow cylinder around it so both belt ends are pressed into the worm gear with their teeth. Now you can turn the worm gear to tighten or loosen the belt "loop". You can 3D print the worm gear and enclosing cylinder to match your belt tooth profile and thickness.