To start an application minimized (like the Todoist snap as in below), create a script similar to the following.
You can add the script to autostart on login in your DE.

#!/bin/bash
# Launch Todoist as a background process
/var/lib/snapd/desktop/applications/todoist_todoist.desktop&
# Loop, waiting for the window to initialise
i=0
while [ $i -lt 20 ]
do
        sleep 1
        # check if the window can be found
        window=`wmctrl -l|grep "Todoist"`
        if [ "$window" != "" ]
        then
                # Found the window
                for w in $window
                do
                        # Minimise the window
                        xdotool windowminimize $w
                done
                break
        fi
done

Source: https://www.reddit.com/r/kde/comments/b0k01e/start_application_at_login_minimized/