Webhook Json Value Cutoff Issue

The payload you're showing here isn't a JSON (application/json) payload.

It's a Form/Url (x-www-form-urlencoded) payload that happens to have a single field (order) that contains stringified and URL-encoded JSON.

As such, you can't simply choose JSON as the request token encoding because the entire JSON block needs to be URL-encoded to transport data without error.

For example, if order is

{ 
  "OrderHeader" : {
     "CustomerID" : "xxx"
  }
}

then this is passed as

https://endpoint.example.com/username=VER&order=%7B%22OrderHeader%22%3A%7B%22CustomerID%22%3A%22xxx%22%7D%7D

You can't do this in a single step. You need to pass the fields to a separate 'hook and get back a single pre-JSON-encoded block, which you then store in a textarea field. Then you send that {{lead.textarea field}} in the payload to your real service, now using Form/Url encoding.