lineItemAdd.cs

// add line item to  Cart
using (var client = new HttpClient())
{  
   // assembledEndpoint: {{host}}/{{projectKey}}/me/carts/cartId
   var endpoint = assembledEndpoint;
   var body = new Dictionary<string, string>
   {
     { "version", "1" },
     { "actions": [{
            "action": "addLineItem",
            "productId": "f5635f84-7c0b-4df7-8ea2-5134911cd5ea",
            "variantId": 1
        }]
     },

   };
   var content = new FormUrlEncodedContent(body);
   var response = await client.PostAsync(endpoint, content);
   var responseString = await response.Content.ReadAsStringAsync();
}