Pressing Enter key with an external keyboard doesn't send SMS on Panasonic P100
The Enter key can produce 10 (hex 0A), 13 (hex 0D) or 10-13 (2 characters, 0A, then 0D), depending on the operating system it was actually designed for.  The SMS app may be looking specifically for 10 (0A), because that's the Linux EOL (end-of-line) code (and Android is running in a Linux OS).  Use the mouse to click the (in most SMS apps) forward arrow on the screen.  (Or try alt-10 - hold the alt key while pressing 1, then 0, in turn, then release the alt key.  That produces the Linux EOL character.)