Edit the LXSession autostart file

sudo nano ~/.config/lxsession/LXDE-pi/autostart

Add absolute path to your bash script towards the bottom of the file.
Bash script must have absolute path refferences like this

/usr/bin/omxplayer /home/pi/wk-portraits-screens/starfield_render.mp4 --loop --no-osd -o local --display=5

No need for sh at the beginning of the line if your script permissions are set to executable.