How can I withdraw my bonus?

The bonus must be completely “worked” in order to be withdrawn. In order to work your bonus you must open trades complying to MTP requirements. (MTP is a minimum distance between the open and close price, and can be checked here.
For those orders you will receive 17.5% from spread and commission (if there is commission on account). Once this amount reaches the credit bonus amount it will be considered completely “worked” and the bonus amount will be credited to your account balance. You will see two operations in your account history — credit and balance operation. Credit operation will deduct the bonus amount from the credit field and the balance operation will add the bonus amount to the account balance. After this, the bonus can be withdrawn.