@Robert Heynen Form Data can be extracted in the set-body
policy as mentioned in the official docs and you should be able to append values to it like so.
<set-body template="none">@{
var formData = context.Request.Body.AsFormUrlEncodedContent();
formData.Add("secret", new List<string>() {"123", "456"});
return formData.ToFormUrlEncodedContent();
}</set-body>
If your backend requires JSON, you can replace the return line with JsonConvert.SerializeObject(formData)
which will output the object as JSON instead.