I think you want to make a simple web page/form, either password protected or not, for your external users to create a new calendar entry. I was just trying to do something similar and found some useful information in the wiki. Here's the link followed by a little of the actual information:
User Migration - ZimbraWiki We've added the ability to update/create content by POST'ing content to REST urls. GETs continue to remain read only, with no side-effects (as they should be).
For example, you can POST an RFC822 formatted message to your inbox REST url to append messages to the inbox folder. Using the popular curl program, this would look like the following:
curl -u schemers
assword --data-binary @/tmp/rfc822.txt https://server/service/home/schemers/inbox
Note, you currently have to use /service/home for POSTs instead of /zimbra/home, because /zimbra/home issues a redirect, which isn't allowed with POSTs. We'll be fixing that in an upcoming release.
Other items that can be updated this way are calendar appointments (ICS), and contacts (csv and the new vcf (vCard) format):