adding extra damge on a natural 20

i have a player using the 5e sheets how would i add a extra damage dice for things such as Expert Slaughterer

Click the gear icon next to the attack entry you want to modify. It will have an empty crit field, empty means it copies the normal damage di(c)e by default. Simpy fill in the crit field with the total extra dice that would be rolled on a crit. So if your normal damage is 1d10 and you have a feature that adds an extra die on crits, you would put 2d10 in that field, so you'd have the default 1d10 plus 2d10 more whenever a crit is scored.

I guess more what im asking they have a crit range of 18-20 they do double but on a 19-20 they do triple is there a way to set it up or do we just need to keep rolling the extra

Create a damage rider mod and have the player only click it when it matters.  You can also make a damage rider into an api button in the description and click it there if they see they rolled in the range.  There is no way to autoroll different damages on different crits in an extended range that I'm aware of.   The image is not of a damage rider but of a button similar to a rider that fires either if they nat 1'ed or nat 20'ed.  My pink text is the modified style of the pink buttons.