You can add this link to the header section from the theme editor.
but since every theme has a different structure for files and section i can't tell you where you can find the code to edit.
i will need to login into the dashboard myself to find it.
it might be in a section, it might be in a snippet or it might be in the main theme file i have to check it out.
and also it could work in the home page as a section but it will require some HTML and CSS skills