Before I answer your question, I'd like to correct you, the throttle control on the bottom middle is also an axis if you must have axis control over something (although I don't recommend it).
If you want a button to move a motor at a set speed when pressed, use something like
this.
If you want a button to move a motor at a set speed when pressed but does not reset,
this will do the trick.
What the first one does is you get the button of the joystick you want to use, and when true, selects the .5 motor value and sets the "reach motor" to run at .5 speed until the person lets go of the button.
The second one, however, just sets the motor to run at .5 speed whenever one button is pressed, and turns it off with another button.
The reason I chose these two examples are to show the multiple ways you can use the buttons to move motors. (using case statements and using selects)