when i place order on sandbox - it gives error "Missing product_id"

[sbadar]

I am able to use all other API functions other than placing a new order.

I am passing parameters as following:
/orders?product_id=BTC-USD&size=1&price=1&side=buy
and every time i get error "Missing product_id"

Any help??

Following is my code (for placing a hard-coded test order)

public async Task<ApiResponse<Object>> PlaceOrder() {
            var response = await this.GetResponseAsync<Object>(
             new ApiRequest(HttpMethod.Post, $"/orders?product_id=BTC-USD&size=1&price=1&side=buy&","")             
            );

            return response;
        }