The following code will convert all form bodies into textareas:

add_filter( 'wpforo_post_after_init_fields', function( $fields ){
    $fields = 'textarea';
    return $fields;
} );

Insert the code into your active theme functions.php file. For more information visit: https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/

Please make sure that all caches are cleared before checking. Also, make sure to click the "Reply" button to view the type of editor.