I'm trying to setup Adobe IO events to post notification on Slack for assets created, deleted and so on. However, OOTB if I do, the payload is not compatible to Slack as it requires to have a mandatory property text for Slack which is not part of Adobe IO event payload.
So, how can I customise the payload to have event posted on Slack?
You cannot directly use a Slack webook with Adobe I/O Events, because of the same constraint you mentioned. You will need a webhook to proxy your event payload to Slack in a compatible format. This can be achieved by a simple Adobe I/O Runtime action (or any other serverless function). Below is a sample repository for inspiration: