Maximilian, thank you very much for your help.

I experimented a little more and also concluded that the method doesn't work for the pulses option. However, I did find a work around.

Instead of using the fast-gpio access through the ubus call (which would allow me access from anywhere), I created a 1-liner script that looks like this:

fast-gpio pulses 19 $1 3

and named it /root/gpio19

Going back to the Onion Cloud, I chose the method launch-process, and as the command I set:

/root/gpio19 /root/input.csv

The response is:

{
"resp": "Command to launch in background: /root/gpio19 /root/1.on.csv"
}

and it actually does what was supposed to!

By the way, I'm using this to create a middle man between the Internet and my RF controlled outlets. I can now develop apps or crontabs to turn things on and off through the house.