little php in e-mail response

Hi folks,

I searched 3 days now for a solution of that problem. I hope you can help me – it seems to be so easy -.-

I want to do a little php script and add the result (string) to the automated email response.

I managed to add the script and the answer in functions.php:

add_action( 'wpcf7_before_send_mail', 'wpcf7_add_text_to_mail_body' );

 function wpcf7_add_text_to_mail_body($contact_form){

     // get mail property
     $mail = $contact_form->prop( 'mail' ); // returns array 

     // add content to email body
     $yourcode = "Your code is: ";
     $yourcode .= "HERE SHOULD BE THE VALUE OF A TEXTBOX";
     $mail['body'] .= $yourcode;

     // set mail property with changed value(s)
     $contact_form->set_properties( array( 'mail' => $mail ) );

 }

The only problem is: How can I generate the $yourcode out of a textbox of the form? How can I access the fields?
Imagine the use fills out the data and types “123abc” in the “yourcode”-Box.
So that to the mail is added: “Your Code is: 123abc”

Thank you so much…