There is a plugin outlet
https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/templates/topic.hbs#L172
If you don’t want to build a plugin for it, go to Admin > Customize > CSS/HTML and put this in the </head> section
<script type='text/x-handlebars' data-template-name='/connectors/topic-above-suggested/additional-content'>
Test
</script>