Add internal Bridge to Hassio internal MQTT

Hi,
I have just got owntracks reporting back to Hassio internal MQTT broker successfully.
However, I already run another MQTT (Mosquitto) broker on a separate RP3 that has been working great with HA for several months now. It is set up without SSL or Usernames and passwords as it sits inside my network.
I have a number of MQTT Clients linked to it as part of my HA set up - sonoffs, Particle Photons, ESP devices. So not super keen to play with the setup to much.
What I was hoping I could do is use the HASSIO MQTT as my external facing MQTT and rout the owntracks request through to my internal MQTT for HA to deal with.
I know this is possible through conf changes on the MQTT server, but not sure how to do this with the HASSIO MQTT.
I tried adding the following to the config through HA like in this post

“bridges”: [
{
“connection”: “mqtt”,
“address”: “internal IP and port”,
“remote_username”: “bridge_testuser”,
“remote_password”: “bridge_testpw”,
“clientid”: “HomeAssistant”,
“try_private”: “false”,
“start_type”: “automatic”,
“topic”: “# in”
}
]

But this just got wiped when i Saved it and restarted Hassio MQTT.

How do I add the bridge to the internal HASSIO MQTT?

thanks

Same issue.
Check how i did it here