The link to the online application can be found on the Open Call website and is the following:
https://synchronicity.submittable.com/submit
You can have a look beforehand at the questions in the application form and some guidelines for answering them in the "Application Form Template" document that can be downloaded from the web under "Open Call Toolkit for SME and Large Business Applicants (or New City Applicants)" (https://www.synchronicity-iot.eu/open-call/).