The WMCH Cronos web page should support some GET parameters to prefill its form:

https://wmch.toolforge.org/cronos/

For example:

• event_date_start: (string) date expressed as Y-m-d

But actually this does not work because if you are not logged-in you lose the GET parameter after being redirected from your OAuth login of Meta-wiki.

The landing URL after OAuth is hardcoded in the configuration and cannot be customized for security reasons, so we have to remember the event_date_start in the session before doing the first OAuth initial redirection to Meta-wiki.