Can anyone help with crit damage?

Hello all! I am trying to make it so that when a player in my game rolls a nat 20 in my 5e D&D game the damage is the result of a damage die times 2 with their modifiers added. Is there a macro or system set up I need to use? An example would be someone rolls a nat 20, they roll a d8 and get an 8 so the critical damage would be 16 plus whatever their modifier is for that attack.

Unfortunately, die rolls cannot be reused.

The 5e character sheets handle critical damage as an added value  (7+2) for example, where 7 is the normal damage and the +2 is from the crit, rather than multiplicative.  You would have to manually multiply it, or at least make a very simple  Total: [[?{Original Damage?}*2]] Macro to make it easier. Other sheets handle it differently, but there's no options as a player to change that behavior.  As Scott C. mentioned, you can't retain a roll's result without making use of an API script, which is only available to GM's with PRO subscriptions.