how do i get the 255 to track my daily HIIT ( non garmin HIIT)

Hi.  Just got a new 255.  Love it. 

I can use it to track my runs etc  but i do a daily HIIT that isnt a Garmin HIIT.  How do i get the 255 to track my activity when doing the 20 min HIIT?

 I dont want to set up a Garmin HIIT activity.

Thanks