Get url of file – step by step

hello, i have a problem with implementing my own function into wordpress. I have this plugin and I need to display / add link to the file uploaded by the form. I used the code sent 2 threads before but I don’t know exactly where to add this code:

// The action callback function.
function example_callback_id_generated( $attachment_id ) {
    // (maybe) do something with the args.
    $url = wp_get_attachment_url( $attachment_id );
}
add_action( 'nmr_create_attachment_id_generated', 'example_callback_id_generated', 10, 1 );

Adding it to the functions.php file in the themes directory I get a fatal wordpress error.

I then added it to the file: store-file-uploads-for-contact-form-7.php, But when calling it as “[nmr_create_attachment_id_generated]” I get plain text – no location ….

I want the file location to show up in the textbox.