library MyLibrary;

{$mode objfpc}{$H+}

uses
  Classes, MainUnit;

exports
  GetCRMUserGuid;

begin

end.

-----------------------------------------------------------------------

unit MainUnit;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, Math;

procedure GetCRMUserGuid(ADBName, AHost, AUser, APass: PChar; var AResult: PChar;
  var AResultLen: Integer); stdcall;

implementation

procedure GetCRMUserGuid(ADBName, AHost, AUser, APass: PChar; var AResult: PChar;
  var AResultLen: Integer); stdcall;
var
  S: string;
begin
  S := String(ADBName) + String(AHost) + String(AUser) + String(APass);
  AResultLen := Min(AResultLen, Length(S));
  StrPLCopy(AResult, S, AResultLen);
end;

end.

-----------------------------------------------------------------------

[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Files]
Source: "MyLibrary.dll"; Flags: dontcopy

[Code]
procedure GetCRMUserGuid(ADBName, AHost, AUser, APass: AnsiString; var AResult: AnsiString; var AResultLen: Integer);
  external 'GetCRMUserGuid@files:mylibrary.dll stdcall';

procedure InitializeWizard;
var
  Buffer: AnsiString;
  BufferLen: Integer;
begin
  BufferLen := 255;
  SetLength(Buffer, BufferLen);
  GetCRMUserGuid('DBName', 'Host', 'User', 'Pass', Buffer, BufferLen);
  MsgBox('We received ' + IntToStr(BufferLen) + ' chars. Here they are: ' + Buffer, mbInformation, MB_OK);
end;