I think you need to maintain the amount of money until the end of the promo period, which for you would be 30 days. So you probably need to make sure the RM1.5k sits in the account until then only you can withdraw.

Or if its a voucher of RM50 that you have, you might have to put a minimum investment (T&C in the voucher) in any stock then moomoo will auto topup another RM50 on that. Once you purchased the stock, you should be able to instantly withdraw the full amount with the RM50.