The key is labeled such that it looks like holding down fn and pressing Return will produce Enter (as with Apple’s keyboards). However, when I do that it only produces a regular Return.
If you don't need to use fn otherwise, you could simply remap it to behave like Enter. This is easily doable using the free tool Karabiner-Elements — download and install it, then add a "Simple Modification" from
keypad_enter. If you want to only remap your external keyboard, then set the checkboxes in the Devices tab appropriately.
If you want to keep fn's functionality, you need a more advanced remapping/macro utility such as BetterTouchTool or Keyboard Maestro. I'm not aware of any free options in this realm (though they may exist) besides the full version of Karabiner, which doesn't currently work on Sierra.
fn is not a typical modifier key, so it can't be captured in the same ways that the others (e.g. ⌥ and ⌘) can. Because of this, there is no way I'm aware of to assign fnreturn to anything. However, using one of the tools I mentioned, you can easily assign another hotkey to do the same thing, for example, ⌥return.
@MichaelTsai I'm betting that this is a Logitech driver issue. I have used the K811 for a couple of years and have had no problem with fn-return as enter (as well as the mappings for fn-arrow keys and fn-delete outlined in this help document).
However, I just recently upgraded to Sierra (currently 10.12.6) and upgraded my Logitech options/firmware (v6.70.1030). Shortly after doing so, I realized that fn-return no longer worked. The other fn mappings still do and, weirder still, all of the fn mappings work perfectly fine using my MacBook's built-in keyboard.
So, I opened up the Keyboard Viewer to see what was going on and the problem became obvious. When typing on the Macbook's built-in keyboard, the keyboard viewer shows the normal keyboard like so:.
When you hold down the fn key, the return key turns into the Enter symbol (⌅) as expect.
However, as soon as you type on the K811, the keyboard viewer switches to an extended keyboard (with numeric keypad):
In this configuration, the fn key has no effect on the Return key's symbol (or, obviously output).
My guess is that the Logitech firmware is incorrectly defining the keyboard type for the operating system. I have filed a support ticket with Logitech (case #06038823) and will let you know what they say.