Publication Manual of the American Psychological Association (7th Edition) by 
APA Format in Word 2016 by Colin Murphy, Ed.D.
DOI is a digital object identifier
The DOI is a a web address linked to the publisher information of the article or book chapter. This is more stable than using a link from a database. A group of publishers, Publishers Linking International Association (PILA) formed the CrossRef depository as a collaborative linking system. To find more information go to https://www.apa.org/pubs/librarians/industry/crossref
To find a DOI for a publication you can use the CrossRef website: https://www.crossref.org/
A simple way to do so is to use the Search Metadata search and enter the title of the article. Then you can check for the matching item.
