I was having the similar issue, getting the same error code. So, I tried to research the documentation and it didn't say much.

Then I tried the following -

{
"json": {
    "code": "hahhaa",
    "site_id": "6249"
    }
}

This was the body I sent if the request is of post type (mutation). Else, If you want to send the values in query parameters for "GET" request (query), then you can do the following -

?input={"json":{"code":"hahhaa","site_id":"4548652"}}

Add the above in the url at the last, obviously replacing the values inside and you are good to go.

Pre-Requisites -

Your trpc.procedure.input code looks something like this -