As can be seen from the screenshot of the widget settings to the right, the button text is entered and then styled for font family, size, color and background color.
The button width is adjusted using a drag handle or the Metrics inspector. The height is adjusted using the value for top/bottom padding. For use as a CTA on a mobile website the button should be at least 30px high.
The button can have a box shadow if required and can also be styled for font color and background color on hover. These colors can be set to the same as the normal colors for mobile use since hover is not supported.