We have a recommendation (APA style) on our homepage:
https://www.soscisurvey.de/index.php?page=info&l=eng (at the bottom)
You may want to or have to quote SoSci Survey as the software you used to collect your data. Strictly speaking, you have to distinguish between the software and the website service with its servers. While a software is quotable (see at the end of the page for a recommended quotation according to APA), the service can only be quoted in-text with its URL. Here is an example as to how to do this:
"The questionnaire was created with SoSci Survey (Leiner, 2014) and made available to the participants on www.soscisurvey.com"
Hint: The software is regularly updated. You will find the current version number after logging in on the left at the page end.
Leiner, D. J. (2018). SoSci Survey (Version 2.5.00-i1142) [Computer software]. Available at http://www.soscisurvey.com