The required elements for the reference are:
Author/Corporate Author. (year of publication) Title. Place of publication: Publisher. Available at: URL. [Accessed date].
Reference List
Example of the full reference at the end of your assignment:
ClinCalc LLC. (2020) Sample size calculator. [s.l.]: ClinCalc LCC. Available at: https://clincalc.com/stats/samplesize.aspx. [Accessed 11 March 2021].
[s.l.] sine loco - means no place of publication
In-text citation
Example 1 - Example 1 - where the author's name occurs naturally in the sentence
ClinCalc LCC (2020) found that using the sample size calculator.....
Example 2 - where the author's name does not occur naturally in the sentence
Recent sample size (ClinCalc LCC 2020)