how to redirect users to a page after login only when a specific menu item is clicked? I have a link "Add a Post" (url: /node/add/mypost) which is not directly accessible to users. 

I want to display a menu item titled "Add a Post" so that when users click on it, user goes to login page and THEN goes to "/node/add/post"

I have tried using "logintoboggan" and "rules" modules but these redirect user to "node/add/mypost" EVERYTIME a user logs in - I also have a "log in" only link where I just want the user to come back to home page after logging in.

Question Summary: How to have redirect users to a node add form based on a menu item that they clicked on? (menu item here = separate than the "log in" link)
You could add the link like this:

user/login?destination=node/add/post


after login they'll be sent to the add post form.