Technically, this does not exist as an advertising objective. However, you can create a Website Clicks campaign with the link on an Instagram page (for example, https://instagram.com/manchesterunited/) and write a call to subscribe to an @account in your text. The Ads will be published from your Instagram account.
Please note that direct links to Instagram accounts do not always work (Error 100). If this happens, insert a link to another website and call to action in your image or text.