This is a little unintuitive
Found from this post
drop a Custom HTML Element on your map
while the element is still marked, look at the top right of the map editor where you can see the properties of the element
Top: Left: Width: Height: Layer: HTML:
use the little arrow at left to HTML
now enter your text in the HTML Before field and save.
now you should see the text in the Custom HTML Element on your map.
<font size=”8″>This is some text!</font>
if you want it to be bold
<b><font size=”8″>This is some text!</font></b>