Well my question is quite simple, I would like to populate a field with the value of a variable that have been passed via a $_GET Request.
thanks
You might want to extend the contact form. I don't know how this works nowadays and how the plugin has changed, but I had to do it in the past and I wrote a blog post about it: http://kovshenin.com/archives/wordpress-extending-the-contact-form-7-plugin/
wpcf7_add_shortcode()
function http://contactform7.com/2015/01/10/adding-a-custom-form-tag/