How can I shoot with my Samsung nx300 in bulb mode, without continuing to press the shutter button?
Generally you should use a remote shutter release to hold the shutter open for a prolonged period. This also prevents camera-shake that is usually associated with touching the camera while taking a long exposure.
Several options are available from simple lockable switches to complex intervalometers.
Each have their own advantages but to get started a cheap lockable remote shutter release would be a good place to start.
To use this, you simply activate the shutter by pressing the button, then you can usually slide it one way or another to lock it in place. This will hold it for as long as you want until you manually release it.