Where do you add code in custom fuctions so it works on a specific page?

You are missing ‘ in your if statement. Then it should work, and you can use comma to specify various IDs or slugs for pages.

if(is_page('health-fitness')) {

Try this.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)