For Mac + iPhone users, it is near impossible to send WhatsApp messages from the desktop. However, there is a trick to make your life easier and write WhatsApp messages using your keyboard.
First, download & setup iKeyboard on your Mac; which is available here: https://itunes.apple.com/us/app/ikeyboard/id441439411?mt=12 . iKeyboard basically lets you use your Mac keyboard as a bluetooth keyboard on any device. You could also find alternative apps doing the same thing, but I’ve been using iKeyboard since a long time without any problems.
After iKeyboard is up & running, connect your iPhone & Mac over bluetooth. At this point; whenever you enter a textbox on your iPhone, you can type in using your Mac’s keyboard as long as iKeyboard has the focus.
From now on, you can open WhatsApp, focus on the message box and type fast & easily using the keyboard of your Mac.
Hint: Hitting CMD + Enter will send the message, so you don’t need to touch the “Send” button at all.