redmine/app/views/issue_relations/create.js.erb

$('#relations').html('<%= escape_javascript(render :partial => 'issues/relations') %>');
<% if @relation.errors.empty? %>
  $('#relation_delay').val('');
  $('#relation_issue_to_id').val('');
<% end %>
$('#new-relation-form').show();
$('#relation_issue_to_id').focus();