Force Mac Mail to send in Rick Text

Is there a way to force an email to send as Rich Text even if the body contains no formatting?

We use Office365 and have 'append disclosure' set with a HTML signature.

In Mac Mail client, I have selected the Rich Text option under 'compose' in the Preferences.

If I send an email with formatted text in the body (font in red), the email is sent as Rich Text and displays the HTML signature as expected.

However, if I compose an email but don't include any formatting in the body, the mail gets sent as Plain Text, which means the signature is appended in plain text and doesn't show the html styling.

Even with the Rich Text option under 'compose' in the Preferences selected, if there's no formatting within the message body it sends as plain text.

How can I force it to always send as Rich Text, even if there isn't formatted text in the body?