app/views/doorkeeper/authorizations/form_post.html.erb
<header class="page-header">
<h1><%= t('.title') %></h1>
</header>
<%= form_tag @pre_auth.redirect_uri, method: :post, name: :redirect_form, authenticity_token: false do %>
<% @authorize_response.body.compact.each do |key, value| %>
<%= hidden_field_tag key, value %>
<% end %>
<% end %>
<script>
window.onload = function () {
document.forms['redirect_form'].submit();
};
</script>