These two theme components may be of substantial use for you, if you’re ok with putting the link you described above in the header:
https://meta.discourse.org/t/custom-header-links/90588/2
Alternately, you can choose to put that link in the hamburger menu where it’s a little less conspicuous by use of this theme component:
https://meta.discourse.org/t/custom-hamburger-menu-links/87644/6