rhodecode/templates/admin/repos/repo_edit_fork.html

${h.form(url('repo_as_fork', repo_name=c.repo_info.repo_name),method='put')}
<div class="form">
   <div class="fields">
       ${h.select('id_fork_of','',c.repos_list,class_="medium")}
       ${h.submit('set_as_fork_%s' % c.repo_info.repo_name,_('Set'),class_="btn btn-small",)}
   </div>
       <div class="field" style="border:none;color:#888">
       <ul>
            <li>${_('''Manually set this repository as a fork of another from the list''')}</li>
       </ul>
       </div>
</div>
${h.end_form()}

<script>
    $(document).ready(function(){
        $("#id_fork_of").select2({
            'dropdownAutoWidth': true,
        });
    })
</script>