The APA citation format for the CertMaster courses is as follows:
TestOut Corp. (Publication Date). Title of CertMaster course (section #.#.#). Retrieved from http://www.testout.com
Example:
TestOut Corp. (2019, January 2). TestOut PC Pro (section 1.2.4). Retrieved from http://www.testout.com
A list of publication dates can be found here. Please keep in mind that if another format is needed, the same information can still be used.