MIDI Start and Stop messages are the standard way to control sequencers. They're their own type of messages, they're not PC, CC or anything else. There are two ways you can get the Mastermind series to send these messages.

If you're using the Mastermind to send MIDI Clock, then you can turn on the "Send Start and Stop" setting in MIDI settings. When the MIDI clock is turned on, it will send a Start message and when MIDI clock is turned off, it will send a Stop message. The Tempo settings in each preset and song control whether the clock is on or off, or you can use a System / BPM action on any IA button.

To send a Start or Stop message manually, you need to use the Sysex message feature, which can send any type of MIDI message. Select a Sysex message, set its name to "Start". In the message data, put the value FA. Select another Sysex message, set its name to "Stop", and in the message data, put the value FC. Remember the numbers of those two messages.

Now, on any IA button, you can add a Sysex action to the button which sends the Start or Stop sysex.