GraphiQL 404 page not found

Question

Based on this Shopify documentation, I'm trying to execute this GraphQL query:

query getDiscount($code: DiscountCodeSortKeys) {
  priceRules(first: 1) {
    edges {
      node {
        discountCodes(first: 1, sortKey: $code) {
          edges {
            node {
              code
              id
            }
          }
        }
      }
    }
  }
}

In GraphiQL in my Shopify admin page I get this output:

{
  "data": {
    "priceRules": {
      "edges": [
        {
          "node": {
            "discountCodes": {
              "edges": [
                {
                  "node": {
                    "code": "discount_code_1",
                    "id": "gid://shopify/PriceRuleDiscountCode/1888888"
                  }
                }
              ]
            }
          }
        }
      ]
    }
  },
  "extensions": {
    "cost": {
      "requestedQueryCost": 6,
      "actualQueryCost": 6,
      "throttleStatus": {
        "maximumAvailable": 1000,
        "currentlyAvailable": 994,
        "restoreRate": 50
      }
    }
  }
}

However, when I execute the same query in GraphiQL IDE after editing the HTTP headers and putting the GraphQL endpoint (https://my_domain.com/api/graphql), I get a html code with the title of 404 page not found as you can see in the picture below:

Answer 1

The problem was in the selected method "GET", it worked when I selected the "POST" method.