5th ed optional critical hit damage

This is bound to have been covered before, but I cannot find any reference. When Roll 20 detects natural 20 on an attack roll, it then rolls two separate damage rolls and shows them, correct? Is there any way of making the second roll just produce the maximum damage that character can cause with the weapon used? Secondly, is there anything already in place that does this or any of the other  critical damage options? I do have Pro subscription level. Cheers :) 

For PCs using the 'D&D 5E by Roll20' sheet, you have access to the crit value for each weapon, but not for NPCs. In these screenshots, the handaxe is on a PC, and there is a 'crit' field. For the handaxe, you would enter a value of '6' instead of leaving it blank (which defaults to 1d6 to match the regular damage roll). You'll have to modify this for each weapon on each character - there's no universal setting for 'max damage on crits'. The morningstar is on an NPC, and there is no 'crit' field.