[Help] [5E] Multiple Damage Dice on specific hit dice rolls

Hello, sorry if this is something simple and I just don't understand how to do it but I am trying to make it so when I attack someone with a weapon a second damage dice will roll only when it is a 17-20  Specific example: Short sword 1d6 and does an additional 1d6 damage when a 17-20 is rolled not including any modifiers (so just the initial hit dice)

Do you mean like a crit? If so, there's a line in the entry when you're editing the attack for crit range. It defaults to 20, but if you change it to 17 then any roll of 17 or higher (before modifiers) will count as a crit and roll the extra damage die for crits.