[Konstantinos]

I don't understand your question.

An example (from a tutorial):

pawn Код:

public OnPlayerConnect(playerid)
{
    if(fexist(UserPath(playerid)))
    {
        INI_ParseFile(UserPath(playerid), "LoadUser_data", .bExtra = true, .extra = playerid);
        ShowPlayerDialog(playerid, DIALOG_LOGIN, DIALOG_STYLE_PASSWORD,""COL_WHITE"Login",""COL_WHITE"Type your password below to login.","Login","Quit");
    }
    else
    {
        ShowPlayerDialog(playerid, DIALOG_REGISTER, DIALOG_STYLE_PASSWORD,""COL_WHITE"Registering...",""COL_WHITE"Type your password below to register a new account.","Register","Quit");
    }
    return 1;
}

extra parameter is assigned playerid which is given by OnPlayerConnect callback.