Turn a load on with a pulse of power, keep on until (NO) button is pressed
Here's a circuit that will work for you:







simulate this circuit &ndash; Schematic created using CircuitLab

When a pulse drives D1 the relay turns on. The first relay switch connects the coil to a +12V rail through SW1, that is a normally closed push button. The second switch gets closed too and can be used for any purpose.

When you want to turn off your rig you simply push SW1 thus disconnecting the relay coil and opening both switches: when SW1 will be released the relay won't turn back on.

D1 is there to prevent current from the +12V to flow back to the pulse generation circuit, that might or might not like it. If the pulse generator circuit is just a momentary switch connected to the same +12V you can safely remove D1.

Please note that I am assuming that the pulse will last long enough to turn on the relay and toggle the switches. some 100ish of ms should be enough. If your pulse is very short, i.e. 1ms or less, you will probably need a more sophisticated circuit.

I know that this topology has a name, in italian it sounds like "relay with auto hold". If someone can point it out I'd gladly add it. Thanks to Passerby I've learnt that this is called (Self) Latching Relay Circuit, that is not a latching relay that normally includes two coils.

edit

I see that in the title you say your switch is a normally open one. The easiest and safest way to use a NO that comes to my mind is to use a NC SPST or a SPDT relay to invert the behaviour of the disengage button as in this answer from Passerby. Or substitute it with a NC button already.