I have created a child ccs file for my style.css and that works fine.
I tried to do the same with my functions.php file, but it crashes the website.
I know with the style.css child I have to put some code at the beginning of the file to link it back to the main style.css Is there specific code to link back to the main functions.php file when I create a child file
The linking to the parent theme is done by the child theme being in a sub-directory of the parent theme. Have you checked out https://codex.wordpress.org/Child_Themes ? Specifically, your child css should be named style.css