In case you have previously received funds from your IB, you will need to refund them to them before making a withdrawal. You can perform the transfer to the IB in your Trader Area’s Finance section.
To do it, click on "Finances" in the menu on the right and choose "Transfer":
In the pop-up window, select “Transfer to Partner”:
Input the amount and click the “Transfer” button.
The transfer will be completed within a few minutes, and you will be able to see it in your Transaction History.