Figured it out. I'll try to explain as I understand it. And my initial question may have been unclear. The reason I was getting the, 'Support user added.' message is because I was updating the wrong method. The method above should have been the :create controller method, not the :new controller method. :new uses HTTP GET, which is why it would go directly to the redirect. :create accepts an HTTP POST. So, instead, I have the following:
def create
begin
user = User.find_by_email(params[:email])
if user
UserPermission.create(:user_id => user.id,
:permission => UserPermission::SUPPORT,
:creator => current_user)
end
rescue ActiveRecord::RecordNotFound
flash[:warn] = 'User not found'
end
redirect_to admin_support_users_path, notice: 'Support user added.'
end
def new
render 'new.html.arb', :layout => 'active_admin'
end
And this correctly creates a nice looking active admin form, accepting an email parameter.