We tie it in a knot. Take the two ends, hold them next to each other, ends pointed the same direction, and tie a single normal overhand knot with both ends at once. That makes a loop. Then anchor the loop at both ends over something round, such as a 1/2" od nylon spacer with a 1/4" bolt thru it holding it to the robot or arm or whatever that end needs to attach to. If you make the anchor end adjustable, then you can adjust the tension as needed.
for example