Seems like it can be tricky to have other fonts than CSS Web Safe fonts. Something that kind of worked for me:
<!DOCTYPE html>
<h1 style="font-family:Poppins, Helvetica, sans-serif;"> Insert your heading here</h1> <p> This is the paragraph for other text.</p>
CSS Web Safe Fonts that should work: https://www.w3schools.com/cssref/css_websafe_fonts.php. I wanted to use Poppins font and it seems to only work in Outlook desktop app, so that is why I have the Helvetica as a back up font.
