In order to stream pre-recorded video via Loola to social channels, you will need to use additional software.
The idea is to load the video in such streaming software and send the output via RTMP (or virtual camera) to Loola.
Some recommended streaming software that can play pre-recorded videos are:
ManyCam: https://manycam.com/ (Windows + Mac)
Ecamm Live: https://www.ecamm.com/mac/ecammlive/ (Mac)
OBS Studio: https://obsproject.com/ (Windows + Mac, Free)
SparkoCam: https://sparkosoft.com/sparkocam (Windows)
CamTwist: http://camtwiststudio.com/ (Mac)