How to execute an xmodmap command on startup

I use Lubuntu 18.04.

I'm trying to assign my keypad delete key to em dash permanently.

AutoKey-GTK doesn't seem to be able to handle this for some reason.

The xmodmap command works, but I'm having difficulty getting it to execute at startup to make it permanent.

I've tried (all without success):

• Adding em dash as a phrase in AutoKey-GTK
• Adding the xmodmap command to the Lubuntu startup programs GUI
• Adding the command to the above as a bash script.
• Creating .Xinitrc and .Xmodmap files as described by another poster here.
• Adding it to the default applications list with a 15 second execution delay.
• Creating a desktop launcher and adding that to the 'autostart' folder.
• Adding the command script to the crontab under @reboot and running every two minutes.

So far nothing has worked.

The command is simply:

xmodmap -e "keycode 91 = emdash"

Anybody have any suggestions?