Base64 encoding of text

Job ID: 33710359

Budget: $30 – $250 NZD

I am wanting to mimic the encoding that a 3rd party application performs on plain text using C#.
Attached is a sample of Base64 encoded text that the 3rd party application produces.

I have managed to decode this to plain text using C#.
Also, I can decode the sample Base64 using the following web page: https://base64.guru/converter/decode/file

However, I cannot figure out the C# code to convert the plain text back into the exact Base64 encoded text.
Hence, I require the C# code to perform this encoding.

If you use the above web page to decode the sample Base64, it returns a zipped file (with name str.dat) that contains the plain text.
Related categories: .NET C# Programming