Within Teamwork.com, you can choose to use markdown instead of the HTML editor for messages and comments in your profile preferences.
To do this, click your profile image to the bottom eft corner of your site and select Edit My Details from the pop-out menu.

Select the Preferences tab from the edit details modal and toggle on the Use Markdown for messages and comments option at the bottom of the modal.

If you have enabled this option, you won't see the formatting toolbar in the editor. Instead, you can use markdown syntax directly in the content to format it.
You can find lots of detail about markdown syntax here, but here's a quick example to show what it looks like. First, here's the code.
This is a message with **bold** text and *italic* text
Here is a list:
- Item 1
- Item 2
- Item 3
Then, this is the resulting formatting that you'd see as a result:
This is a message with bold text and italic text.
Here is a list:
- Item 1
- Item 2
- Item 3
Note: If you edit a message that was created with markdown originally, you won't see the editor even if you haven't enabled markdown in your profile preferences.
You can also use the GitHub Markdown cheatsheet for further detailed information
For more information, see: Updating Your Profile Preferences