As explained before, you can simply link to a url. So, to open an article in the modal window, you can use the url of the article, like:

{modal url="sample-articles/my-article"}Link to an article{/modal}

Change the catid and id values to your actual category and article id.

Quick links PRO

However, with the Pro version of Modals you can link directly to a Joomla article without having to know the exact url. You can simply place the article's title, alias or id, with the article attribute.

{modal article="My Article"}Link using article title{/modal}
{modal article="my-article"}Link using article alias{/modal}
{modal article="371"}Link using article id{/modal}