1. 确定 GA4 跟踪代码中已定义了自定义维度。
2. 获取 Google 客户端库，并使用以下代码获取自定义维度的值：

// Replace with your own client ID and API key
const CLIENT_ID = 'YOUR_CLIENT_ID_HERE';
const API_KEY = 'YOUR_API_KEY_HERE';

// Replace with the actual property ID and custom dimension index
const PROPERTY_ID = 'YOUR_GA4_PROPERTY_ID_HERE';
const CUSTOM_DIMENSION_INDEX = 'YOUR_CUSTOM_DIMENSION_INDEX_HERE';

// The Google API endpoint to call
const API_ENDPOINT = 'https://analyticsdata.googleapis.com/v1alpha:runReport';

// Request body that defines the report query
const requestBody = {
  "entity" : {
    "property_id" : PROPERTY_ID
  },
  "dimensions" : [
    {
      "name" : "customEvent:custom_dimension_" + CUSTOM_DIMENSION_INDEX
    }
  ],
  "dateRanges" : [
    {
      "startDate" : "today",
      "endDate" : "today"
    }
  ],
  "metrics" : [
    {
      "name" : "sessions"
    }
  ]
};

// Make the API call using the Google APIs client library
gapi.client.request({
  'path': API_ENDPOINT,
  'method': 'POST',
  'headers': {
    'Authorization': 'Bearer ' + access_token
  },
  'body': requestBody
}).then(function(response) {
  console.log(response.result.rows[0].dimensionValues[0].value);
}, function(reason) {
  console.error('Error: ' + reason.result.error.message);
});

3. 运行代码并查看控制台输出，以获取自定义维度的值。