want to assign social media logos with links in my apple mail 7.3 at the signature level

Using Apple Mail 7.3.

I want to make my signature have the social media logos clickable to my social media page. I've been reading online select image assign in drop down menu link but my drop down menu link is grayed out? Anyone know how to do this or is it doable in this version of mail? My OSX version is 10.9.5