That is what I did. I added an extra tick box with 80k next to it as the one on the website only has
Marathon du Mont-Blanc (42km)
Cross du Mont-Blanc (23km)
10km du Mont-Blanc
KM Vertical
I am not sure anyone looks at them as your entry gets a green tick as soon as it is uploaded but best get it right. There is a max upload file size.
I am pleased there will be some other entrants from the UK for the 80k. My club had 7 runners in the Cross this year (plus me in the marathon) but I could not find much enthusiasm for the 80k next year.