class Program {
    static async Task Main(string[] args) {
        // get file to encode
        using
        var file = new FileStream(@ "C:\Users\fodoo\Desktop\logo.png", FileMode.Open);
        // copy file to Memory
        using
        var ms = new MemoryStream();
        await file.CopyToAsync(ms);
        //setup configuration for the encoder
        var config = new WebpConfigurationBuilder().Output("output.webp").Build();
        // create an encoder and pass in the configuration
        var encoder = new WebpEncoder(config);
        // start encoding by passing your memorystream and filename
        var output = await encoder.EncodeAsync(ms, Path.GetFileName(file.Name));
        /* your converted file is returned as FileStream, do what you want download,
           copy to disk, write to db or save on cloud storage,*/
        Console.WriteLine($ "Your output file : {Path.GetFileName(output.Name)}");
        Console.WriteLine($ "Length in bytes : {output.Length}");
    }
}

public async Task < IActionResult > UploadAsync(IFormFile file) {
    if (file == null) throw new FileNotFoundException();
    //you can handle file checks ie. extensions, file size etc..
    //creating output file name
    // your name can be a unique Guid or any name of your choice with .webp extension..eg output.webp
    //in my case i am removing the extension from the uploaded file and appending
    // the .webp extension
    var oFileName = $ "{Path.GetFileNameWithoutExtension(file.FileName)}.webp";
    // create your webp configuration
    var config = new WebpConfigurationBuilder().Preset(Preset.PHOTO).Output(oFileName).Build();
    //create an encoder and pass in your configuration
    var encoder = new WebpEncoder(config);
    //copy file to memory stream
    var ms = new MemoryStream();
    file.CopyTo(ms);
    //call the encoder and pass in the Memorystream and input FileName
    //the encoder after encoding will return a FileStream output
    //Optional cast to Stream to return file for download
    Stream fs = await encoder.EncodeAsync(ms, file.FileName);
    /*Do whatever you want with the file....download, copy to disk or
      save to cloud*/
    return File(fs, "application/octet-stream", oFileName);
}

// create your WebP Configuration using fluent builder
var configuration = new WebpConfigurationBuilder().Output("image.webp").Preset(Preset.DEFAULT).QualityFactor(200).AlphaQ(10)
//.... add more advanced options//
.Build();