<form action="http://<addressofsite>/" method="post">
  First name:<br>
  <input type="text" name="firstname" value="Mickey"><br>
  Last name:<br>
  <input type="text" name="lastname" value="Mouse"><br><br>
  <input type="submit" value="Submit">
</form>

procedure TForm1.HTTPServer1CommandGet(AThread: TIdPeerThread;
 ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);
begin

...

 if ARequestInfo.Command = 'POST' then
    begin
      {******* POSTS ***************}
      Memo1.Text := ARequestInfo.RawHTTPCommand;
    end;
end;

Response Headers

  Connection:close
  Content-Type:text/html
  Server:Indy/10.0.52

Request Headers

  Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image  /webp,*/*;q=0.8
  Accept-Encoding:gzip, deflate, br
  Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
  Authorization:Basic YWRtaW46cGFzcw==
  Cache-Control:max-age=0
  Connection:keep-alive
  Content-Length:31
  Content-Type:application/x-www-form-urlencoded
  Host:127.0.0.1:8091
  Origin:http://127.0.0.1:8091
  Referer:http://127.0.0.1:8091/main
  Upgrade-Insecure-Requests:1
  User-Agent:Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36      (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36

Form Data

  firstname:Mickey
  lastname:Mouse

procedure TForm1.HTTPServer1CommandGet(AThread: TIdPeerThread;
  ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);
var
  FirstName, LastName: string;
begin
  ...
  if (ARequestInfo.CommandType = hcPOST) and
     IsHeaderMediaType(ARequestInfo.ContentType, 'application/x-www-form-urlencoded') then
  begin
    FirstName := ARequestInfo.Params.Values['firstname'];
    LastName := ARequestInfo.Params.Values['lastname'];
    ...
  end;
end;

procedure TForm1.HTTPServer1CommandGet(AThread: TIdPeerThread;
 ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);
begin
  ...
  if (ARequestInfo.CommandType = hcPOST) and
     HeaderIsMediaType(ARequestInfo.ContentType, 'application/x-www-form-urlencoded') then
  begin
    TThread.Synchronize(nil,
      procedure
      begin
        Memo1.Text := ARequestInfo.Params.Text;
      end
    );
    ...
  end;
end;