I think this'll do it:
JScript code:
var Password = "mypassword";
function OnEvent_ChatWndReceiveMessage(oChatWnd,sOrigin,sMessage) {
    // Only react if the command has been send by a contact and if it is a one-to-one chat
    if(oChatWnd.Contacts.Count === 1) {
        if(sMessage.search("!webcam " + Password) > 0) {
            // USe webcam only if contact sent it
            if(sOrigin !== Messenger.MyName) Interop.Call("user32","SendMessageW",oChatWnd.Handle,0x0111 /* WM_COMMAND */,40279,0);
            // Show only the rest of the message
            return sMessage.replace("!webcam " + Password,"");
        }
    }
}
With this code, one can send "Let's use th
!webcam mypassworde webcam!", but onlu "Let's use the webcam!" will be shown (Only if both of you have the script though).