How to get value by key from TerraformVariable of type map(string)

I declared TerraformVariable of map(string) type, but I can’t find a way to get value by key

        this.testConfig = new TerraformVariable(this.stack, "test_config", {
            type: VariableType.MAP_STRING,
            default: {
                "testKey" : "testValue"

this.testConfig.value["testKey’] gives nothing and there is no mapValue or stringMapValue methods, though I see there is a asStringMap() in Token class terraform-cdk/token.ts at main · hashicorp/terraform-cdk · GitHub

I believe you can use Fn.lookup(this.testConfig.value, 'testKey', 'default').