There is no way to do what you want using Wait blocks - the wait blocks will only wait for a time period. We do have on our roadmap to enable “wait for a webhook” but we don’t know when that will be ready.
What you can do instead is split your flow at the point where you are going to wait for the webhook, and then create a new flow (after the webhook is received) with a Webhook trigger. Then you can proceed with however you want the conversation to flow after that action.
The trick there is that your webhook needs to provide a “response_path” parameter as part of its data payload that you can use to set the response_path on subsequent actions in the flow.
The reason is that Webhook triggered flows don’t know what user they should be associated to, so you have to tell it that information.
You can find the response paths for users in the interaction logs and you can use the action menu in the Users area to copy a users response path, or you can send it to your server in a flow using HTTP so that it knows how to respond to any given user.