How to clear a String from memory properly? - delphi

Question

I have followed those questions and answers to try to clear a String from memory:
Delphi : how to completely remove String from the memory
Creating string from byte array
In my case I did something like this:
function generateEncryption(Textdata : String) : String;
var
SaltB, IVb, Padtext, keybytes, datas, SaltIVMessage : TBytes;
Cipher: TDCP_rijndael;
keysize, TextDatasize, saltivmsglen : Integer;
OutData, keygen : String;
begin
keygen := 'MyTargetedKey';
keybytes := TEncoding.UTF8.GetBytes(keygen);
Cipher := TDCP_rijndael.Create(nil);
try
cipher.CipherMode := cmCBC;
cipher.Algorithm := EncryptionType;
cipher.BlockSize := 16;
Cipher.Init(keybytes[0], keysize, #IVb[0]);
TextDatasize := high(PadText) + 1;
SetLength(datas, TextDatasize);
Cipher.EncryptCBC(PadText[0], datas[0], TextDatasize);
saltivmsglen := Length(SaltB) + Length(IVb) + Length(datas);
SetLength(SaltIVMessage, saltivmsglen);
SaltIVMessage := AddSaltIVMessage(SaltB, IVb, datas);
OutData := TNetEncoding.Base64.EncodeBytesToString(SaltIVMessage);
Result := OutData;
finally
cipher.Burn;
cipher.Free;
// No success
FillChar(keybytes, Sizeof(keybytes), 0);
// No success either
//ZeroMemory(#keybytes[0], Length(keybytes));
//FillChar(keybytes, Length(keybytes), 0);
//keybytes := nil;
// Still no success
//UniqueString(keygen);
//ZeroMemory(Pointer(keygen), Length(keygen)*SizeOf(Char));
//keygen:='';
end;
end;
I used the GitHub repository "strings2" to extract the String from the executable to read its Strings in the memory. I can see MyTargetedKey as following:
:TNetEncoding.:2
System.NetEncoding
0123456789ABCDEF
MyTargetedKey
In my code I tried different things without success (see comments). What is the proper way to totally clear a String from memory to avoid the capture of my app key, or how to make it harder to capture it?