On the diagram I have for the F-18 there is a three-position switch located on the right side of the throttle labelled "communications". The positions are 1, off & 2.
(from F-18 Hornet in detail & scale, ISBN 0-8168-5016-X).
The F-15 is probably the same since it was the first application of HOTAS, & McDonnell had a big hand in F-18 development after Northrop lost the FSX competition to General Dynamics.
Usually the button is on the throttle (closest to the pilot on a twin engine) positioned where you can press it with your thumb when your hand is on the throttles. I would imagine it's a fairly standard position - the T-37 & T-38 I flew a hundred years ago had it positioned there, and it just makes sense, to have it right at hand like that.
It is the same in the Boom Pod on the KC-135. The buttons are on the Ruddervator stick, but two seperate buttons, one for the radios, and one for intercom. The KC-135 Pilots had a three position switch on the yokes, intercom, neutral, and radios.