void registerMiniAppApi(String apiName, TcmppMiniAppApiHandler apiHandler)

final _tcmppFlutterPlugin = TcmppFlutter();

...

@override
void initState() {
   super.initState();

   ...

   /// Must register before the mini program starts up
   _tcmppFlutterPlugin.registerMiniAppApi("myApiName",myApiHandler);
}

example code:
   /// Mini Program Call
 var opts = {
 api_name: 'myApiName',
 success: function(res) {
   log(res);
 },
 fail: function(res) {
   log(res);
 },
 complete: function(res) {
   log(res);
 },
 data: { 
     name : 'kka',
     age : 22
  }
 }
 wx.invokeNativePlugin(opts); 

/// Client APIs
Future<Map<String, dynamic>?> myApiHandler(MiniApiCall call) async {
    print("API : ${call.apiName}");
    print("AppInfo: ${call.appInfo}");
    print("WebView ID: ${call.webViewId}");
    print("params: ${call.params}");

    return {"result": "success","method":"myApiHandler"};
}

class MiniApiCall {
  /// Mini program information
  AppInfo? appInfo;
  /// The current custom API name
  String apiName;
  /// The WebView page ID
  int webViewId;
  /// Request parameter information. Example: {stateEvent: requestCustomApi, api_name: customApi, data: {param1: test, param2: 189}, webviewId: 4}
  Map<String, dynamic>? params;
}