How do you activate the portal in battleunderb in AQWorlds?

complete the girl's quests in battleunderb until you reach the final quest, which is opening the portal. to open the portal, a shadowy shape will appear on some sort of rune thingy, and it will disappear quickly. you have to draw a line on where the shadowy shape was from one end to another, and dont let a single part of the line get out of the shape.