how to send 26 byte hex value using textbox through serialport(rs232)/rs422

Looking at this from the point of view of conversion of a string in hex format to byte[] and conversion of byte[] to hex format string with the requirement to optionally use a format where hex bytes are separated by dashes:

private static string dash = "-";

private static byte[] ByteAryFromHexString(string hx)
{
    // has dashes ? delete them
    if (hx.Contains(dash)) hx = hx.Replace(dash, "");
    
    // unnecessary ?
    hx = hx.ToUpperInvariant();
    
    byte[] result = new byte[hx.Length / 2];
    
    for (int i = 0; i < result.Length; i++)
    {
     result[i] = Convert.ToByte(hx.Substring(i*2,2), 16);
    }
    
    return result;
}

// the 'withDash parameter controls whether dashes are in the result : it's set to 'false by default
private static string HexStringFromByteArray(byte[] bytes, bool withDash = false)
{
    var result = BitConverter.ToString(bytes);
    
    if (withDash) return result;
    
    return (result.Replace(dash, ""));
}

Test:

string hx1 = @"AA-55-55-55-15-55-6A-AA-FF-FF-00-00-FF-FF-00-10-00-06-08-8F-00-E2-A0-80-08-D3";

string hx2 = @"AA55555515556AAAFFFF0000FFFF00100006088F00E2A08008D3";

private void Test()
{
    // set break-point on next line
    // and single-step through the code

    byte[] b1 = ByteAryFromHexString(hx1);
    byte[] b2 = ByteAryFromHexString(hx2);

    // do b1 and b2 have identical values ?
    string enc1 = Encoding.UTF8.GetString(b1);
    string enc2 = Encoding.UTF8.GetString(b2);
    bool i1 = enc1 == enc2;

    string s1 = HexStringFromByteArray(b1, true);
    string s2 = HexStringFromByteArray(b2, false);

    // s1 and s2 should not be the same !
    bool i2 = s1 =!= s2;
}

Note: Hex format String to Byte[] conversion is one place .NET doesn't really offer a really fast built-in conversion. There is a great deal of debate about the fastest way to achieve that: you can find extensive discussions, and performance measurements, of various methods on threads on StackOverFlow like this one: [^].