If you are asking about Comcast's "Appsuite" webmail, in the Compose window you can click Options and select HTML to access the formatting bar for that message.
You can also change your settings so that HTML is always selected. From the main webmail screen click
(gear icon) / Settings / Mail / Compose / Format emails as
and select "HTML" or "HTML and plain text".
Note that if the Compose window is not "Maximized", some of the format bar items will be missing. See https://forums.xfinity.com/t5/Email-Web-Browsing/Appsuite-webmail-v7-10-0-1-3/m-p/3291863#M214496.
If you are not asking about Comcast's webmail, what program or app are you using?
@Sansby wrote: ... Are you saying there is another setting to change? ...
Using webmail on a laptop or desktop, yes. Using the app on a mobile device, apparently not. I can find no way to do it using the Connect App on my cheapo LG phone. The phone's native email client doesn't have HTML compose capability either.
I can't find a clear statement on whether the Connect App allows HTML formatting, but this section from https://www.xfinity.com/support/articles/xfinity-connect-mobile-app-faqs suggests that it doesn't:
Can I edit an email when trying to forward/reply?
- For HTML emails: While replying/forwarding an HTML email ... The app will warn you that "Editing the text will remove all formatting. Do you want to continue?" Tap YES to open the original message without the HTML formatting.
- Currently, editing of HTML email format is not supported. All HTML emails will be converted to plain text before allowing editing.