Hello.

Webster's shows "onboard" and "on-board" as an adjective or an adverb.

American Heritage shows "on "onboard" and "on-board" as an adjective but "onboard" as an adverb.

Looks like you get to take your pick!

Hey, Richard.

"For clarity . . ."

For clarity, my friend, it simply doesn't matter. On this, as with so many other matters, the authorities do not agree, so we are free to follow wherever our preference leads us. Whether it's Merriam-Webster's Unabridged, Webster's New World, or Random House Webster's Unabridged, again, it is of no importance whatsoever.

Have a fine weekend, Richard.

I understand exactly what you mean, Richard, and it was a response that shows you are someone who realizes not all Websters are created equal. Merriam-Webster is certainly the authority I turn to.  Can't just trust any so-called dictionary--and I try to steer clear of those that have the word "College" in the title or, God forbid, "New World."  Nope, I don't subscribe to anything's acceptable just so long as it's in print and calls itself a dictionary--but that's just me.  Make it Merriam.