How to add in a Horizontal Rule in Upfront?

Hi Team,
Just a small quick question that I apologise in advance if its rather noobie.

I just can't seem to insert a horizontal rule into my posts when using upfront.
If I add one in the traditional way through the WP backend, it will show up in the WYSIWYG editor, but as soon as its published, it seems that Upfront removes it. So I tried wondering around inside Upfront to see if I can insert one here, but I can't seem to find anything.
I apologise if its right before my eyes and I'm missing, if so can you please point it out to me?

Otherwise, suggest ways I could add one in?

Thank you kindly team for your continued support with all my WP efforts :slight_smile: