A DOI, or digital object identifier, is a unique, permanent identification number that will take you straight to a document no matter where it is located on the Internet. You can find out more about DOIs in the 7th edition of the Publication Manual of the American Psychological Association (pp. 298 –300). DOIs figure prominently in the APA 7th edition referencing style, and where a book, journal, report or other publication has a DOI, it must be included in the reference.
DOIs must always be presented in the format https://doi.org/xxxxxx, for e.g. https://doi.org/10.7748/phc.2016.e1162.
It is also acceptable to use http://doi.org/xxxxxx (rather than https). However, if the item you are referencing has a DOI in a format other than these, it will need to be changed to the above format. It is a good idea to check that the link works correctly before submitting.
Note that a DOI will usually link to a record on a publisher's website, and may not always include full text, even though the Library may have full text access. It's always worthwhile checking the Library catalogue or databases & e-journals page to see if full text is available.