See title

Using the tools on this site no,this site does not even support existing epic boons at the moment,and will not be able too for a few months more.

Depending on what you want the boon to be, you might be able to homebrew a feat or a magic item that would do the trick. 

The current work around is to make a Feat that has the effect of an Epic Boon.

if you want to take it multiple times ctrl c ctrl v.

Reiterating that epic boons (and the mechanically similar dark gifts) are currently not a function in D&D Beyond.  However, post VRGtR, there's more impetus for D&D Beyond to implement a system and they've stated they've intended to do so within the year.  This system would presumably also enable the boons described in Theros and suggested in the DMG (though I don't remember whether DDB has explicitly stated that or if its community inference).

Till then, some folks use feats as a workaround, my inclination is to just do a write up in the sheet's notes and manually modify or acknowledge in my offline game long accordingly when used.

It's worth noting that mechanically, there's not really any difference between an 'Epic Boon', a 'Dark Gift', a 'Supernatural Gift', or anything else and a feat.  They're all the same thing - a discrete ability or package of abilities added to your character outside of its class progression.  It's why I tell folks to simply homebrew feats and use the out-of-progression Feat management system in their sheets - there's no real difference whatsoever between any of these things and a 'feat', save that feats can (sometimes) be taken in place of a class-granted ASI while these things usually cannot.  The homebrew feat system can usually (not always, but usually) simulate these things just fine.