So Ok... drill not too deep small hole in the correct place of the fuselage.Then apply with a needle a very small drop of a glue to the hole.Or if you want , on one of the ends of the thread. But I suggest testing which one glue ( a glue for plastic or cyanoacrylate Superglue) is better for the nylon thread.Simply try to stick the thread to a piece of mould frame.
Then put the thread into the hole and wait for glue hardening. Of course the wire from the fin to the mast had to be sticked before.Now stretch the thread and add a drop of a glue at the place where both wires crosse.Wait for a while for hardening of the glue.Using scissors or a razor blade or a sclapel cut off the excess of the thread.