Are you going to make a story or you already have a story? Well, it's easy. Just go to the chapter of the/your story that you want to dedicate to someone. Then, at the upper part of the book/story cover, there are those 'writer tools': "Edit", "Dedication", "Video", "Photo" (If you want to edit the chapter, dedicate it to someone, add a video or photo).
Click the word "Dedication" then a choice box will pop-out (if you want to dedicate to a Wattpad member or what/who-so-ever). Click"Wattpad Member" and type/search for the username of the person you want to dedicate your story to.
I hope these instructions help you! :D
PS. Please keep on supporting to my stories! Thank you!