Figured it out. In Automator, I created a workflow, then dragged in "Run Shell Script" into the edit area (large grey box on the right). Then I entered this line into the script:
/Applications/Organteq\ 2/Organteq\ 2.app/Contents/MacOS/Organteq\ 2 --fullscreen
and saved it as a workflow in the Applications Folder as 'Organteq_full_screen'
Then go to 'System Settings --> Login Items and Extensions' and add the file 'Organteq_full_screen' to the 'Open at Login' list.
Not sure if this is the approved way, but it seems to work as I want it to. If anyone knows a better way I am all ears.