Add a POI while running

Look at the Save Location feature on your watch.  I've setup a hot key to quickly save waypoints/locations either during activities or just from the watch face.

I would use the Garmin Explore App to sync your saved locations and even your Spartan activity.  The waypoints can be easily edited in Explore (name, icon, location).

You can also use Garmin Basecamp to do the same thing I just described with Explore.  Basecamp is a computer only application, however.

Saving the current position is indeed possible as Dan.Boilermaker wrote, and you do not even need setting up a hot key - just long-press the GPS button to get to the GPS menu, where the position saving is available rather easily.

Another option is pressing the lap button (if enabled), or tagging the lap through the Pause menu - that has the advantage that you do not need Garmin Explore for viewing the POIs, and can see the lap positions directly on the activity map in Garmin Connect. The lap functionality can be also assigned to a hot-key that works even when the lap key (Set/Back) is disabled in given activity profile.

Dan.Boilermaker said:

I would use the Garmin Explore App to sync your saved locations and even your Spartan activity.

Below is an example of a previous run activity and saved waypoints that were sync'd to Explore from my f7.  I changed the default waypoint icons and names in Explore after the sync.  trux's method is the only way I know to tag a location to a saved activity within Garmin Connect.  You can't save single locations in Connect.

Dan.Boilermaker said:

trux's method is the only way I know to tag a location to a saved activity within Garmin Connect.

Well, there is another option, or kind-of. If you do not wish to add supplementary laps to your activity (not to mess up the auto-laps), you can run a few steps to the side and back at your POI, creating a side-tick on the track which will allow you to see it on the activity map in Garmin Connect. Though, that's not something you are likely to use at a race, I guess

Can you elaborate on the “GPS button?”

Might be labeled the Start/Stop on the Epix. Upper right. 

On Epix 2 it is called Start/Stop (right up). No sure though now whether it has the same functionality as on other models. May be that you finally do need to assign the hotkey, unlike with some other watches. More details on it here: epix (Gen 2) Owners Manual - Saving a Dual Grid Location (garmin.com)

kenxxxxxx said:

Can you elaborate on the “GPS button?”

I think this might be an Instinct function. The top right button on the Instinct (2) is labelled GPS and log press of that brings up a GPS menu. I've not used it so cannot elaborate more at the moment.