Are you trying to add a new webhook after deleting one? That URL is the correct one. Here is the sample curl command to POST a webhook URL:
$ curl --request POST
--url 'https://api.twitter.com/1.1/account_activity/all/env-beta/webhooks.json?url=https://www.kylesAwesomeSite.com/webhook'
--header 'authorization: Bearer TOKEN'
It is important to note, you can only have a single webhook URL per environment. If you are still running into an errors, please be sure to post the error message that is being returned as well as the full URL you are calling.