Dynamics contacts app ID
Open any contact in Dynamics and look at the address bar. The URL includes an appid parameter:
https://myid.crm.dynamics.com/main.aspx?appid=123a321a12-aa11-xy12-ab12-12332112332f&pagetype=entityrecord&etn=contact&id=123a321a12-aa11-xy12-ab12-98778998778
The value you want is the appid, so in this example 123a321a12-aa11-xy12-ab12-12332112332f. Note that the URL also carries an id parameter, which is the contact's own ID and not what you're after.