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