Closing a position means repaying what you borrowed. In the margin order form, keep the trading mode set to Normal and place a regular order in the opposite direction of your position.
How it works
To close a long, sell the asset. To close a short, buy back the asset you owe. When the order fills, the proceeds automatically go toward repayment: interest first, then the borrowed amount. Whatever is left is yours.
What to keep in mind
This option gives you the most control: you choose the order size, type, and price. But you also need to size the order yourself. If your order is smaller than what you owe, part of your position stays open and keeps accruing interest. If it's larger, you may end up opening a position in the opposite direction.
Example: you are long 0.03 BTC and owe 2,000 USDT plus interest. You place a sell order for 0.05 BTC, more than you hold. The proceeds cover your interest first, then the 2,000 USDT you borrowed, and the remaining USDT stays in your Margin Wallet. But since you sold 0.02 BTC more than you had, Bitso automatically lent you that amount: your long position is closed, and you've opened a 0.02 BTC short position, which also accrues interest until you close it.