ActivityFunction.cs

[FunctionName("CreateMessage")]
public static async Task<string> FirstActivityFunction([ActivityTrigger] string input)
{
    return $"Hello {input}!";
}

[FunctionName("SendMessageToQueue")]
public static async Task SecondActivityFunction(
    [ActivityTrigger] string input,
    [Queue("messages", Connection = "StorageConnection")]
    IAsyncCollector<string> messages)
{
    await messages.AddAsync(input);
    await messages.FlushAsync();
}