You can't. It is just a textarea HTML control that provides no facility for formatting text.

Are you hoping to have something like the WhatsApp way of doing it?

To italicize your message, place an underscore on both sides of the text: _text_
To bold your message, place an asterisk on both sides of the text: *text*
To strikethrough your message, place a tilde on both sides of the text: ~text~