how to use wordpress default shortlink for sharing

Same question here.

And beyond… as I would like to attach custom UTM parameters to the link and then make a shortlink out of it, but that might be a second step (if possible).

Not easily done right now, but it is planned.

If you write JavaScript, AddToAny events could help you make it happen in the meantime:
https://www.addtoany.com/buttons/customize/wordpress/events

Thanks, got this to work.

Now appending UTM parameters, then creating a bit.ly shortlink. Example:

var a2a_config = a2a_config || {};
a2a_config.callbacks.push({
    share: function(share_data) {
        // Parameters
        var params = '?utm_source=source&utm_medium=medium';
        // The shared URL
        var old_url = share_data.url;
        // Initialize new shared URL
        var new_url = old_url;
        // Add parameters if not already added to the shared URL
        if ( old_url.indexOf(params, old_url.length - params.length) === -1 ) {
            new_url = old_url + params;
        }
        // Modify the share by returning an object
        // with a "url" property containing the new URL
        if (new_url != old_url) {
            return {
                url: new_url
            };
        }
    }
});
a2a_config.track_links = 'bitly';
a2a_config.track_links_key = 'addtoany|R_cc6a4017b0274d1d86f2397771b70136';
a2a_config.templates = {
    twitter: "${title} ${link} via @twitter-username"
};

But this means my bit.ly credentials are now accessible (in the example it’s from addtoany)…

Any chance you could show how to do this via functions.php instead? Or should I open a new question/thread for this?

Thanks a lot for your support!