In short. The same way you'd do so normally.
In long. It depends on where/how you're attempting to place this button and if you mean an actual button or not.
If you intend to use the StoryMenu special passage, which displays links within the UI bar that are styled to look like the default buttons, then you may simply use the normal link markup or the <<link>> macro. For example:
[[The text|passage name]]
<<link [[The text|passage name]]>>
/* Any code you need run when it's clicked. */
<</link>>
If you intend to use one of the other special passages that display within the UI bar—e.g. the StoryCaption special passage—then you may use the normal link markup, the <<link>> macro, or the <<button>> macro, if you needed a real button. For example: (only showing <<button>> here, the other two are unchanged from the above example)
<<button [[The text|passage name]]>>
/* Any code you need run when it's clicked. */
<</button>>