Step 1: Ensure that you can send direct messages to your server's twitter account. Just follow the server's account and your server's account also needs to follow you.
Step 2: Add the following in your .myst.conf file below MYST section:
You can add more twitter handles separated by blank space.
Step 3: Now you can DM any command to your server's account and it'll be executed. You need to ensure that myst.py script's "getdm" option is in cron.
MYST uses python's os.system to execute these commands. The user's access level who executed myst.py will decide if the commands coming from DM will be executed or not. If you want everything to be executed then run myst.py with super user or using sudo.