How do you release adrenaline at will?

It's not like that. Adrenaline (epineprhine) is a neurotransmitter (chemical which helps nerves communicate). It's release comes from nerves which are part of the autonomic nervous system. The word autonomic means "independent", meaning it works in the background. As such, it only activates or changes in response to events which occur to you. One does not "will" the release of adrenaline.
Why do you want to? First of all, in the US there's no such thing as "adrenaline." It's called that in the UK but here it's called epinephrine. ALWAYS. Second, you're probably laboring under the peculiar delusion that an "adrenaline rush" is a pleasurable, exhilarating feeling. It isn't. Epinephrine secretion is part of the "fight-or-flight" response that is elicited by fear or danger. It's NO FUN AT ALL.