Phoenix.LiveView.Helpers.form/1, which is imported by default into your templates by way of your app_web.ex is what you want to use here. So update your heex template to:
<div class="flex mt-6 mb-8">
<.form let={f}
for={@changeset}
phx_submit="move_items"
phx_target={@myself}>
<div>
<%= select f, :category_id, @category_select, prompt: "Move to list" %>
<%= submit "Move" %>
</div>
</.form>
</div>
