For both? one of the two already has a webcam for this to make sense, so you'll only want it to appear on a single side. I guess you'll just need to send the proper data over the protocol, something which can't be done natively or even with with regular scripting, but not impossible.
Why would you want to though, if you want to know if the have a came just ask them. That will give you a more reliable than Messenger can do automatically, since users can untick the option.
And if it is the link you are interested in, just point them to the Logitech site
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