How do I add margin to a quote?

Open the Quote tab and scroll to Margin. It is between your sections and what the quote comes to.
Pick how you price it. A percentage is the usual way. It lets you re-price a whole quote in one keystroke. Use a lump sum when you decided the number, not the rate.
Or price it to a total. Type the figure before GST that you want, and YourCA works out the percentage that gives it. It stores the percentage, not the figure, so it can always work the number out again.
Use your own name for it: Margin, Overheads and profit, Preliminaries and margin. It is your letterhead and your word.
A negative percentage is a discount. Change the label to Discount to make the document say so.
Decide whether the builder sees it. The tick is off by default, because what you make is your business.
If it is hidden, the exported quote says nothing about margin. YourCA marks up your printed figures instead, so the document still adds up to its own total.
The rate column comes off a hidden-margin quote. A rate times a quantity would no longer equal the amount beside it.

Worth knowing

The margin lets you price your lines at what the work COSTS you. YourCA then turns that into what the builder pays. GST is charged on the marked up price, never on the cost base. When the builder accepts, YourCA spreads the margin across the lines of your contract value schedule. It is not a row of its own, because you cannot claim forty per cent of a margin.