How to use the @grafana/runtime.config.defaultDatasource function in @grafana/runtime

const queries = targets.map(q => {
      if (q.datasource === ExpressionDatasourceID) {
        expressionCount++;
        return {
          ...q,
          datasourceId: this.id,
          orgId,
        };
      }
      const dsName = q.datasource && q.datasource !== 'default' ? q.datasource : config.defaultDatasource;
      const ds = config.datasources[dsName];
      if (!ds) {
        throw new Error('Unknown Datasource: ' + q.datasource);
      }
      return {
        ...q,
        datasourceId: ds.id,
        intervalMs,
        maxDataPoints,
        orgId,
      };
    });
    const req: Promise = getBackendSrv()