<button class="claim-button"> <a href="<?php echo CFS()->get( 'tracking_url' ); ?>">Claim Bonus</a> </button>

<button class="claim-button"><?php echo CFS()->get( 'tracking_url' ); ?></button>

<button class="claim-button"> <a href="<?php echo get_field('tracking_url'); ?>">Claim Bonus</a> </button>

/*
    Returns:
    array(
        'url' => 'http://google.com',
        'text' => 'Visit Google',
        'target' => '_blank'
    )
*/

$link = CFS()->get( 'tracking_url');
<button class="claim-button"><?php echo $link['url']; ?></button>