Modify CF7 Data Before Submission

functions.php

<?php

add_action( 'wpcf7_before_send_mail', 'wpcf7_custom_data', 90, 1 );  
function wpcf7_custom_data( $WPCF7_ContactForm ){

  // Submission object, that generated when the user click the submit button.
  $submission = WPCF7_Submission :: get_instance();

  if ( $submission ){
    $posted_data = $submission->get_posted_data();      
    if ( empty( $posted_data ) ){ return; }
    
    // Got ip data
    $ip_data = $posted_data['ip'];

    $ipv6 = $_SERVER['REMOTE_ADDR'];

    // Got e-mail text
    $mail = $WPCF7_ContactForm->prop( 'mail' );

    // Replace "[ip]" field inside e-mail text
    $new_mail = str_replace( '[ip]', $ipv6, $mail );

    // Set
    $WPCF7_ContactForm->set_properties( array( 'mail' => $new_mail ) );
    
    return $WPCF7_ContactForm;
  }
}