The Victron BMV series Amp Hour Meters have a programmable relay output on the back of the meter. You can program it to operate based on voltage, state of charge, temperature, or other parameters. The procedure is detailed in the instruction manual
starting at page 23. Additional help is available in the Victron community forum
This is the back of the meter:
You can see the relay contacts are set up with common, normally open and normally closed. The alarm can be set to go off with all kinds of parameters but for most purposes you will probably choose either state of charge or battery voltage. If you set the alarm to go off when the voltage gets down to a certain level or based on state of charge then you would use the Common and the Normally Open terminals as your relay output.
The NO and COM terminals are acting as an on off switch, ON when the alarm is active and OFF when it isn't They are not set to carry a lot of current, the amount allowed is specified in the specifications attached to the instruction manual. Here is a screenshot:
If you are using a 12 volt supply then you have to protect it with a 1 amp fuse, you can send it to the COM port and then the NO port will be the signal to turn on your generator. If the generator draws more current on its on/off circuit than 1 amp then your fuse will blow, and in that case you need to put an intermediate relay, so the BMV turns on a relay which sends the signal to your generator. You can Google solid state 12 volt DC relay
to find something suitable.
What happens after that is up to the generator and how it is set up to be activated by a remote start signal.