The APAStyle blog post explaining how to format that and give some background. Basically it describes two possibilities for formatting: Either use the doi with a prefix and colon, e.g. doi:10.1037/rmh0000008, or use the doi in a form of a url, e.g. http://dx.doi.org/10.1037/rmh0000008. However, the prefered form of such an url by doi.org is a shorter variant, namely http://doi.org/10.1037/rmh0000008. This form is also accepted for APA as the following response from the official APA style support confirms:

This DOI world is constantly changing, and APA formulated its guideline to use the http://dx.doi.org prefix based on the recommendation of CrossRef.org. CrossRef still specifies that this is the format to use; I am unaware as to whether they know that the doi.org site suggests a simpler format. Certainly as long as the DOI resolves you have satisfied the purpose of the reference, and in the future I wouldn’t be surprised if all publishers officially endorsed the shorter form. In the meantime, if you want to be a stickler for the rules, the dx.doi format is the one that’s officially endorsed.

Best regards,

The APA Style Expert Team

Because APA has indicated that the shorter form is acceptable, and it is preferred by doi.org (http://www.doi.org/doi_handbook/3_Resolution.html#3.7.3), we chose to go with “http://doi.org/” for the Zotero style.

See https://forums.zotero.org/discussion/47374/doi-in-apa-6edition/#Item_5