sdk-for-dart/docs/examples/functions/create-variable.md

import 'package:dart_appwrite/dart_appwrite.dart';

Client client = Client()
    .setEndpoint('https://<REGION>.cloud.appwrite.io/v1') // Your API Endpoint
    .setProject('<YOUR_PROJECT_ID>') // Your project ID
    .setKey('<YOUR_API_KEY>'); // Your secret API key

Functions functions = Functions(client);

Variable result = await functions.createVariable(
    functionId: '<FUNCTION_ID>',
    key: '<KEY>',
    value: '<VALUE>',
    secret: false, // (optional)
);