There are no strict requirements related to citing Chemicalize. Please mention that the data were calculated using ChemAxon's Chemicalize platform and also include the date of calculation, as well as a reference in a format that is appropriate in the publication (as a bibliography entry or in footnote), e.g. like this:
"Chemicalize was used for [prediction of xyz properties/etc], [month and year of use], https://chemicalize.com/, developed by ChemAxon".