Ruby on Rails: How to validate if on specific page?

In my Ruby on Rails application I am trying to add in validations that will ensure the user has entered a value in a text box. In my system I have a table called Account which stores users' email account information, when they go onto the views/accounts/_form.html.erb page to add a new email account I want to validate the presence of a port number. I can do this through the following code:

validates :port, presence: true

This successfully ensures that users enter their port number, but when a user signs up through the views/users/_new.html.erb page they have to enter only an email address (e.g example@example.com) and the users_controller will then create a record in the Account table for this email address. My problem is that on the views/accounts/_form.html.erb page the port number is required but on the views/users/_new.html.erb it is not.

Is there a way of validating that the user enters the port number if they are on the views/accounts/_form.html.erb page or invoking the create method in the accounts_controller?

I am aware that I could do this through the HTML required validation like so: <%= f.text_field :port, :required => true %> but I need to add in further validation as well as presence, so this is not suitable.