When a user is registered on my website www.xepoulima.com, the website sends him/her a letter with credential details and the website link http://xepoulima.com/login/ to enter ther website and posts his ads.
By ewntering the username and password, the screen that the user sees is the http://xepoulima.com/wp-admin/ with restricted access as contributor and not the screen where he can proceed with submitting his ad, ie filling the forms (title, category, price etc).