Move new subscriber from MailChimp to HTTP Request Node

Check the example below. I believe this is what you want to achive

{
  "nodes": [
    {
      "parameters": {},
      "name": "Start",
      "type": "n8n-nodes-base.start",
      "typeVersion": 1,
      "position": [
        250,
        300
      ]
    },
    {
      "parameters": {
        "requestMethod": "POST",
        "url": "https://webhook.site/c5d689e3-06da-4f5b-9ed3-e167200b7690",
        "options": {},
        "bodyParametersUi": {
          "parameter": [
            {
              "name": "email",
              "value": "={{$node[\"Function\"].json[\"email\"]}}"
            },
            {
              "name": "customFields",
              "value": "={{$node[\"Function\"].json[\"customFields\"]}}"
            }
          ]
        }
      },
      "name": "HTTP Request",
      "type": "n8n-nodes-base.httpRequest",
      "typeVersion": 1,
      "position": [
        700,
        300
      ]
    },
    {
      "parameters": {
        "functionCode": "return [\n  {\n    json: {\n      \"email\": '[email protected]',\n      \"customFields\": {\n      \"firstname\": 'Jim',\n      \"lastname\": 'Miller'\n      }\n    }\n  }\n]\n"
      },
      "name": "Function",
      "type": "n8n-nodes-base.function",
      "typeVersion": 1,
      "position": [
        460,
        300
      ]
    }
  ],
  "connections": {
    "Start": {
      "main": [
        [
          {
            "node": "Function",
            "type": "main",
            "index": 0
          }
        ]
      ]
    },
    "Function": {
      "main": [
        [
          {
            "node": "HTTP Request",
            "type": "main",
            "index": 0
          }
        ]
      ]
    }
  }
}