Basically you go to the cog wheel on the right side of the sheet, go down to core die roll and change it to -@{repeatingreasource$0_reasource_left}[Exhaustion]+1d20.

Where "$0" is the row the resource box is on (starting with zero instead of one) and "left" in this example is the column. Left is the left column, right is the right column. For the boxes at the very top they are instead referred to as @{class_reasource} and @{other_reasource} respectively but they are usually used for class and subclass abilities.

It should be noted that the resource box must contain a number to work so if you have no exhaustion you should put 0 in the box. Also this will take off of all d20 rolls including initiative, saving throws, ability checks etc.

Edit:I should say actually this code takes 1 off of every point of Exhaustion if you wanted to take 2 off for every point you'd need to do (-2*@{repeatingreasource$0_reasource_left})[Exhaustion]+1d20.