How can I add a "Call To Action" for a post using Facebook Graph API? Is it possible or is it free? Thanks in advance.
This is explained very well in the official docs: https://www.facebook.com/help/312169205649942
...or in this article: https://www.facebook.com/business/news/call-to-action-button
Afaik there is no way to do this with the Graph API right now.
Edit: My bad, i thought it would be about that call to action button in the Page header. It is definitely possible to programmatically add a call to action button to a Page post, you just need to include a link to the Page post. I´ve tried it in the API Explorer, just replace the Page ID and make sure you are using a Page Token with the publish_pages permission:
https://developers.facebook.com/tools/explorer/?method=POST&path=[PAGE-ID]%2Ffeed&version=v2.5&message=test&link=http%3A%2F%2Fwww.devils-heaven.com&call_to_action={%22type%22%3A%22SHOP_NOW%22%2C%22value%22%3A{%22link%22%3A%22http%3A%2F%2Fwww.devils-heaven.com%22}}
It´s specified here: https://developers.facebook.com/docs/marketing-api/mobile-app-ads/v2.5#cta_definitions