I am creating an Arcade attribution express to use in Text content in a pop-up configuration. I would like to underline and change the font size of the title (normal text). I am imitating the many examples I've seen online but the attribute expression (attempting underline, only, first) just shows this:

Below is the code I am using
Var Fields = [
"ID: " + $feature.COID + TextFormatting.NewLine,
"Damage Present: " + $feature.DAMAGE
];
If (DomainName($feature, 'FLOW') == 'No') {
return "<UND>" + "Title" + "</UND>" + TextFormatting.NewLine + Concatenate(Fields);
} else {
return $feature.WEATHER24;
}
Any thoughts??
Thanks!