Below example takes input from user in a .Net 6 Console App and appends each input to a text file on BlobStorage using AppendBlob. The connectionString is a SAS for access to the Blob Storage and should be managed in appSettings.json file.

using Azure.Storage.Blobs;
using Azure.Storage.Blobs.Specialized;
using System.Text;

Console.WriteLine("please enter text to add to the blob: ");
string text = Console.ReadLine();

await AppendContentBlobAsync(text);

Console.WriteLine("done");
Console.ReadLine();


async Task AppendContentBlobAsync(string content)
{
    string connectionString = "xxxx";
    string filename = "test.txt";
    string containerName = "mycontainer";
    BlobServiceClient _blobServiceClient = new BlobServiceClient(connectionString);
    BlobContainerClient container = _blobServiceClient.GetBlobContainerClient(containerName);
    await container.CreateIfNotExistsAsync();
    AppendBlobClient appendBlobClient = container.GetAppendBlobClient(filename);
    if (!await appendBlobClient.ExistsAsync())
    {
        await appendBlobClient.CreateAsync();
    }
    using (MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(content)))
    {
        await appendBlobClient.AppendBlockAsync(ms);
    }
}