Hi, from what I understand you have a custom field, let's call it 'cp_example' that users complete with text (it's a text input) and you want to add "http://www." in front of the value that the user inputs. Is this correct? if this is what you want you can modify /classipress/includes/forms/step3.php on top of the file you can put a function that will save the meta_value of the custom field in a variable, add the desired text and overwrite the database input with the new value. custom function would be something like:
function url_custom_field($post_id) {
$old_value = get_post_meta($post_id, 'cp_example', true); //where 'cp_example' is the custom field you want to use.
$new_value = "http://www.";
$new_value .= $old_value;
$wpdb->update('wp_postmeta', array('meta_value' => $new_value), array ('post_id' => $post_id, 'meta_key' => 'cp_example') );
}
I didn't verified the code but it should work all you need to add in the step3.php file is the call of this function with the post id parameter!
Hope it will work as is if not, just contact me, I'm more then happy to help you set it up correct if I miss-understand the request cler things up and we will see what we can do for you.
Regards