Use custom field value as href

To retrieve a value after using add_settings_field() you should use get_option().

So you could do something like this…

$value = esc_html(get_option('option_name'));

if ($value) {
    echo '<a href="' . $value . '">Click Here</a>';
}

You just need to change option_name to your field name. The code will grab the value and if it isn’t false echo the link. Also added the escaping.

*Updated after OP gave more details