Content slide permalink or custom field

Hi,

I’m using the Content Slide plugin which automatically creates a link to the post it is displaying using the permalink. What I’d like to do is create the option to use a custom field in place of the permalink if a custom field value exists.

The code in Content Slide looks like this currently:

<a href="<?php the_permalink(); ?>" title="<?php the_title();?>" <?php echo $new_window;?>><?php the_post_thumbnail ( array($wpcs_settings['width'], $wpcs_settings['height']) ); ?>
        <?php if($wpcs_settings['show_excerpt'] == 'true') { ?>
            <span><h4><?php the_title(); ?></h4><?php wpcs_content_slider_limitpost($wpcs_settings['char_length'], "" ); ?></span>
        <?php } ?>
        </a>

How do I add an if that says, if the custom field $link is not blank, use it instead of the permalink?

Thanks.