Adding Button to Header
Navigate to your styles section in your Studio main menu.
Under the "Header navigation" section, Click on the "STYLE" drop down and change it to the Primary or Secondary button option.
Lastly, keep in mind that any desired changes to the header button text will need to be made through the Header/Footer section.
Button Styles
Changes to the button style, color, and more can be changed in your styles tab under the "Buttons" category.