I just completed something similar --- and the main reason I got a drone in the first place. I'm a ham operator (N4ZAW), interested in getting my communication antennas as high as possible. My intent is to string PowerPro 50# fishing line over trees and other structures as "messenger lines" for temporary HF wire antennas, in as little time as possible. The messenger line is attached to paracord, or nylon UV-resistant rope, to feed it over the support. It werks great when I use a potato-launcher, but i needed more accuracy and range.
This is a 'werk-in-progress', but this what i came up with so far;
This is a simple remote switch system I purchased off of ebay for about $5-US;
I opted to use this device powered directly off the bird's main battery. This is because I didn't want any more servo current going through the Pixhawk MFC --- controllers send funky commands when they are overtaxed with servo demands (well ---- that, and I'll admit, I don't know how)

. So I modded it's 12V output with a standard LM7805 voltage regulator chip (the black 5VDC IC to the left of the blue relay), to power the servo directly through the remote, all powered by the 12V 3S battery. This just garauntees that the Iris+ will not fall out of the sky when I send the "release" command.
This pic shows the simple servo with the signal/pulse board ripped-out that serves as my line release;

It's a HiTec HS-45HB feather servo with the sig board ripped-out, and motor wired direct to the switch output.

Pic above shows the keyfob (comes with the $5 remote switch). It is used to send the release command. The tested range is a little over100 yards, ground-level, line-of-sight.
ISSUES as of now;
Everything is tie-wrapped to the drone at-present, and I might just leave it like that, for quick removal. I have two Iris+ drones, but don't know if I want to commit one to "antenna-stringing" only. It is also reluctant to fit in the box with that gizmo attached to the leg.
Weight. This remote switch has a heavy 10A relay on it that is overkill. The drone seems to have no issues with the added load, but I do...
Handling: It does handle very oddly, as the line feeds-off the spool, and pulls the drone off-center balance. I'm new at this drone stuff in the first place, and my normal flying sucks, much less, this added parameter. i need more time at the football field practicing before I try to use this over trees.
