C# Windows Forms Using FFMPEG to change video format got no response

Question

I am trying to make an exe program to change a video format by using FFMPEG instead of doing it in the terminal. The formatted video will be saved in the download folder. I have tried my code below and got no output response. I wonder if I used process() and StartInfo correctly, as examples I found and the documentation just confused me. I have double-checked the ffmpeg.exe is in the bin folder and the StartInfo() is just for getting information, which is under Process(). This is why Process() can access the information and use Start() to start the process. Please help and correct my understanding. Below is part of my code:

Output: "myprogram.exe(CoreCLR: clrhost): Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\6.0.15\System.Diagnostics.Process.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled." The thread 0x79e0 has exited with code 0 (0x0).

private void convertButton_Click(object sender, EventArgs e)
    {
        String input = filepathTextBox.Text;
        String outputResolution = resolutionLabel.Text;
        String output;
        String outputFileType;
        int inputLength = input.Length;
        int l = 0;
        for (int i = (inputLength - 1); inputLength > -1; i--)
        {
            if (input[i] == '.')
            {
                l = i;
                return;
            }
        }
        output = input.Substring(0, l - 1);
        outputFileType = input.Substring(l + 1, inputLength - 1);
        Process process = new Process();
        process.StartInfo.UseShellExecute = true;
        process.StartInfo.FileName = "ffmpeg.exe";
        process.StartInfo.WorkingDirectory = @"C:\Users\User\Downloads\ffmpeg-2023-05-15-git-2953ebe7b6-full_build\bin";
        process.StartInfo.Arguments = "ffmpeg - i " + @"C:\Users\User\Downloads\1280_10MG.mp4" + " - s 320x240 - r 25 - b:v 500000 - pix_fmt yuv420p - c:v libx264     -vprofile baseline - level  2.1 - x264opts  stitchable = 1:level = 3.0:keyint = 15:ref= 1:merange = 16:mvrange = 32 - acodec pcm_s16le - ar 16000 - ac 1 " + @"C:\Users\User\Downloads\440.mp4";
        process.Start();
    }