[text* text-21]
add_filter( 'wpcf7_form_elements', 'imp_wpcf7_form_elements' );
function imp_wpcf7_form_elements( $content ) {
$str_pos = strpos( $content, 'name="text-21"' ); // тут название твоего input
if ( $str_pos !== false ) {
$content = substr_replace( $content, ' data-attr="custom" data-msg="Foo Bar 1" ', $str_pos, 0 );
}
return $content;
}