If You know the value of first_name, pass it as a variable in the controller…

:first_name is only an atom, pass it, and use it as @first_name.

If You want to use conditional rendering on the same form… You will need some JS skill… or LiveView, or Drab, or any server side libs that can potentially update DOM.

Thanks, that makes complete sense - I was thinking that :first_name would auto-magically be available when the form ‘is loaded’

If you are wanting :first_name to pull from a key on your changeset/whatever named first_name then you can always do this:

<%= form_for @changeset, @action, fn f -> %>
  <%= text_input f, :username, placeholder: input_value(f, :first_name) %>
  ... 
<% end %>

And that will compile the value in as the placeholder. If you want it to update ‘on the page’ then that’s a Javascript thing, so either a some javascript, drab, live_view, etc…

Thanks! Very helpful indeed, input_value is easier than passing variable into a controller