I'm not just talking about sending in Whatsapp. I'm talking about the native texting system for andriod phones. I have a MX Key, and for some odd reason. No matter what I do, I can't get "enter" to "send" a text message. Instead, it just starts a new line.
Wow, this is 8 years old, and nothing has changed. Tech companies can be nasty sometimes. So here is the only 2 correct answers (Also thank you David Metcalfe):
The application may provide an option to use enter as send, but if it doesn't, then I think you can't do much about this...
Press the TAB/right arrow key until the send button of the app is selected. Then hit ENTER. That works for me. Often, the TAB/right arrow key only needs to pressed once or twice before the send key can be activated using ENTER.