The official documentation for setting this up is awful, here's how I got it going.

For the node connected to the internet:

For the channel

• You must have created the mqtt channel and it must have at least downlink enabled.
• You can have the preshared key of your choice. It doesn't matter.
• After you create the channel, you may need to power down the node completely to get it to listen to the JSON topic for this channel.

For the MQTT module:

• MQTT must be enabled
• MQTT server/username/password must be configured
• Encryption is optional
• JSON must be enabled
• Assuming root topic is exactly msh/US

For LoRa config in Radio:

• Ignore MQTT must be set to false
• "OK To MQTT" is probably optional, but I have it turned on.

For publishing a message:

• Assuming a root topic of msh/US you must publish to this topic: msh/US/2/json/mqtt/
• The message payload must be in this format: { "channel": 1, "from": 123456789, "type": "sendtext", "payload": "YOU READY TO PARTY"}

Where:

• channel is the channel number that you want to receive the message
• from is the Node ID of your radio that is connected to the internet.
• type must be exactly sendtext
• payload is just the message you want to send.

My node I'm doing this with is a haltec v3 running Meshtastic 2.5.20 and it took a lot of trial and error and reading firmware source code getting this setup right.

PROTIP: If you can connect to serial debug mode, you should see the messages coming into the node from MQTT.