Q. How do I format an in-text citation in APA for an article by six or more authors?

According to the APA Manual, 6th Edition, page 175:

"When a work has six or more authors, cite only the surname of the first author followed by 'et al.' and the year for the first and subsequent citations."

Example:

According to Smith et al., citations can be fun and rewarding (2019). 

Another great resource to use when formatting citations is Purdue OWL. Here is a link to the page on in-text citations for author/authors in APA.