I know what the command is for the autothrottle to be set to a button on a controller.  

LVAR ONLY… L:HJET_THROTTLE_MAN … 0 = FMS , 1 = MAN

I just cannot figure out how to insert the command when setting it up.  The AAO manual does not help my aged mind.  Help please about setting up new LVAR commands.

There are several ways to do this.

What kind of hardware button is that? Momentary (=toggle) or switch (=On/Off)?

I would wrap the LVar in RPN scripts. (the other way is to let AAO scan for LVars in the sim and then assigning the LVars directly, but that will only give you the ON/OFF option for a switch, not the toggle that you need for a button)

Toggle script:
1·(L:HJET_THROTTLE_MAN,·Number)·-·(>L:HJET_THROTTLE_MAN,·Number)

ON Script
1·(>L:HJET_THROTTLE_MAN,·Number)

OFF Script
0·(>L:HJET_THROTTLE_MAN,·Number)