Such formatting is done with HTML tags in emails and web pages. But fileMaker's send mail generates a plain text email, not HTML formatted email. I don't think it is possible with Send Mail. You can, however, do this with an email plug in that supports html formatting. There are a number of such available and you can web search and evaluate them if you care to acquire one for use in your database.
Thanks so much for the comment. I will go search plugins