Prevent encoding in #FormUrlEncoded retrofit - spring

Question

I need to make a post request such that the json data passed, has key pair value,where the key is data.
For example, if i use jquery as below, data is the key.
$.ajax({
url: “/hello",
data:{ data: JSON.stringify(dataObject) },
type:"POST",
dataType:"json"
});
which is equivalent to
data:{ data: {“firstName”:”john”,"lastName”:”doe”}}.
I am using java spring retrofit, to accomplish the similar. In order to pass the key, i am using #FormUrlEncoded and #Field("data") TypedString userInfo
My retrofit interface is as below
public interface userInfoService {
#FormUrlEncoded
#POST(“/hello")
public Response userInfo(#Field("data") TypedString userInfo);
where TypedString is {"firstName":"john","lastName":"doe"}
However, the request looks something like this
data=%7B%22firstName%22%3A%22john%22%2C%22lastName%22%3A%22doe%22%7D, since it form url encoded.The 3 party api can see the data key, but since it has "=" instea of ":", it cant be parsed
How, do i get the request to avoid = and actually send a decoded request.
I have also tried to use #Body instead of #FormUrlEncoded and #Field. Where #Body has TypedString as {data: {“firstName”:”john”,"lastName”:”doe”}}, but for some reason the 3 party api is not able parse the data key.