A textbox can be added to the Dashboard as a Text tile. Text tiles can be used for displaying static text, such as a header or section label. Creative use of the tile border properties can yield tile groups.
To add a Text tile to a Dashboard, review the Dashboard Designer article. Tiles can be added from the 
Controls for building and interacting with a Text tile are contained in the Tile Properties Pane on the right side of the screen. Click on the tile to select it and the Tile Properties Pane will fill with the properties for the selected tile.
Once added to the canvas, text is entered directly into the Text tile by clicking on it and typing into it.
When a Dashboard is exported as a PDF Snapshot (v2021.2+), text tiles are included in the output.
The Content tab is not available for Text Tiles.
The Style tab for Text tiles is further broken down into Tile Fit, Text Attributes and Background and Border sections.
Fit contains two choices to change how the text fits the tile.
Text contains the standard text formatting controls to change font type, size, alignment, styling and color of the text contained in the Text tile. Checking the Wrap Text checkbox will cause text to wrap to a new line if it is longer than the available width. If unchecked, the text will appear on one line.
The Tile section is where tile appearance can be changed. The settings here override the Canvas Format defaults.
Note
The Tile Header options are only shown if the Tile Header is visible on the canvas.
The Advanced Tab is not available for Text tiles.