Once you have initiated a withdrawal, the transaction is automatically created and sent to the blockchain. Please note that it might take some time until your transaction is confirmed by miners. We are not able to affect the speed of this process as it depends on the current network load.
If you change your mind about withdrawing, you can still cancel the transaction while it has not yet been sent to blockchain (i.e. doesn't have a transaction hash).
If you don't see this option, the transaction has already been sent to the blockchain and is currently being processed.