As far as I know, you can't dedicate a chapter to someone on the Wattpad app so you have to go to the website version of Wattpad.

When you log into your account on the web, go to your works and select a story. Click on the story part you would like to add the dedication to and click on the three dots on the corner of the page.

Click add beside 'Dedicate'.

Type in the username you want to dedicate the chapter to and then press dedicate.

P.S: If you're going onto the web version of Wattpad on your phone, scroll down to the bottom of the page and press 'View Full Site'.