Dynamically Toggle Visibility of a form not showing

Hello community!

I am creating a form in which the user has to select an specific location. However, in order not to show all the locations at once, I would like to show it progressively in two levels:

1- City - User chooses his/her city 

2 - Specific location - Only locations that are located in the previously choosed city show up. 

I have followed the following steps trying to create this configuration: Dynamically Toggle Visibility of a Form Field - Marketo Docs - Product Documentation 

However, when doing a test, the field doesn't appear in the form. 

Can anyone help me please?

Thank you!