Hello @SergioC,
Thanks for sharing your question with the Klaviyo Community!
Since it sounds like you were looking to build out your own custom Unsubscribe or Manage Preference page using HTML, I would advise working with a developer to custom code the sort of dropdown menu you were looking to implement. If you aren’t familiar with a developer, you can find one from our many accredited Klaviyo partners at our Partner’s Directory who can further assist you in.
To add a drop-down menu to the example code provided in the How to Custom Code an Unsubscribe or Manage Preferences Page Help Center article you mentioned, you’ll essentially want to build out the dropdown menu using HTML and align each selection to be passing your desired profile property. You can learn more about custom profile properties and mapping them from the Guide to Properties Help Center article.
I hope this helps!
David