When the temperature on the dial is not set to the steam zone (2 dot or above), your iron will not be hot enough to steam. This can cause its soleplate to leak when you activate steam boost.
Please make sure that the temperature is set to the steaming zone on the dial (indicated in blue in the image) before activating steam boost. Then let the iron heat up in the heel-rest position for one minute before you resume ironing.
Note: The image is for illustration purposes only and may differ from the dial of specific models.