APA 7th Edition Style Guide

A DOI, or Digital Object Identifier, is a string of numbers, letters and symbols used to
uniquely identify an article or document, and to provide it with a permanent web address (URL).

Where can I find a DOI?

In most scholarly journal articles, the DOI can be usually be found on the first page
either below the title or in the header or footer. 

What if I don't see the DOI?

Sometimes, databases like EBSCO don't list the DOI in the record. That doesn't mean
there isn't a DOI for that article! If you don't see the DOI, look up the article on CrossRef.org.
Use the Search Metadata option to check for a DOI for that article.