At budget summary, click Add Charge
Click on Summary (1) at navigation area to show the Additional Charges.
There, click Add Charge (2) to add a new surcharge.

Edit Additional Charges
You can change the description, base and percentage of your additional charge.
Base means on what amount your charge is beeing calculated. Most likely you will use the built-in formula NPC to refer the Net Production Costs, but you can also refer subtotals, sums of tagged accounts or assigned producers.
In this example we want to have a 10% Markup (1)
on net production costs of 1.2 mio € (2),
an amount of 123k €,
leading to a Grand Total of 1.4 mio € (3):

Other examples
To surcharge a fixed amount, set percentage to 100% and base to the desired amount (1)
You can also use formulas, e.g. to calculate a 2% technical insurance on all budget accounts with the tag "insuranced" (150.000), resulting in 3.000 €. (2)
