If you are using API and need to send a "+" sign in the content, you need to triple encode the + sign in the message content.
Here is a sample code for PHP:
<?PHP
$message = "YOUR+++MESSAGE+++++WITH++++
$message = urlencode($message);
$message = urlencode($message);
//now the message is double encoded
//send this message
?>
You can use this for encoding -https://meyerweb.com/eric/tools/dencoder/