[Plugin: Posts 2 Posts] Which api to use for connecting when front end posting?

I used to be able to use p2p_connect to link a post I created on the front-end to the page the new post form was on. Now it generates a fatal call. Which api should I use now?

Using this: p2p_connect($pid, $project_id);

Generates this: Fatal error: Call to undefined function p2p_connect() in wp-content/themes/mytheme/content-page.php on line 65

https://www.ads-software.com/extend/plugins/posts-to-posts/