Please kindly add a navigation collapse element to the header. Then go to the Toggle tab of the options of that element:
Set the width and height of the toggle elements to look like a button:
Set the Border radius option to 0:
If you are interested add a solid border to the button.
Enable the text of the button and add whatever you want as the button text:
Disable the graphics setup:
Now you will have a navigation collapse element that the toggle of that element will look like a button: