It appears that the RED plug is a DIAGNOSTICS plug and thus not likely intended for source of switched power. This is based on information derived from the service manual ( see comments below) I DO NOT recommend to use the plug for powering anything.
It seems that there is another switched power source under the seat. The red coloured plug is easily identifiable. I haven’t yet measured and verified this, but folks at ADVrider have apparently already succesfully tapped into this power source. The other plug is located under the left front fairing. Read more here -> https://honda250rally.com/2017/03/21/honda-250-rally-aux-power-source/
UPDATE I uncapped the plug and gave it a closer look. It has 4 wires going in it. Yellow, green, and 2 black&blues. The plug is indeed a source for swithched power. Depending on how you hook up the wires it seems you can get 12 volts from the green and black&blue or 4,5 volt from the green and the other black&blue. I wish I could get my hands on the service manual and see the electrical schematics.