How to call oracle stored function from visual basic

Dear experts,
I am trying to call oracle stored function from vb6.

Here is oracle function code:

CREATE OR REPLACE FUNCTION "GENERATE_NUM" (
         current_date tbl_message.FLD_CREATE_DATE%TYPE
)
return varchar is
         num number;
         id varchar(13);
         today_date varchar(8);
         cur_date varchar(8);
         temp varchar(20);
         cursor runnum is
                       select to_char(fld_date,'YYYYMMDD')
                    from tbl_runnum
                    where to_char(fld_date,'YYYYMMDD') = to_char(current_date,'YYYYMMDD');
begin
       cur_date:=to_char(current_date,'YYYYMMDD');
       open runnum;
       fetch runnum into today_date;
       if runnum%found then
             select fld_num into num
            from tbl_runnum;
            num:=num+1;
            update tbl_runnum
            set fld_num=num;
       else
              num:=1;
             temp:=to_char(current_date,'MM/DD/YYYY');
             update tbl_runnum
             set fld_date = to_date(temp,'MM/DD/YYYY'),
                    fld_num = num;
             today_date := cur_date;
       end if;
       close runnum;
       id:=today_date || lpad(to_char(num),5,'0');
       return id;
end;
/

This is my vb code:

    Dim objConn As New ADODB.Connection
    Dim objCmd As ADODB.Command
    Dim parm As ADODB.Parameter
    Set objCmd = New ADODB.Command
    objCmd.CommandType = adCmdStoredProc
    objCmd.CommandText = "POSTMAN.GENERATE_NUM"
   
   
    Set parm = objCmd.CreateParameter("PARAM1", adDBDate, adParamInput, , Format(Date, "MM/DD/YYYY Hh:M:SS   AMPM"))
    objCmd.Parameters.Append parm
    Set parm = objCmd.CreateParameter("PARAM2", adVarChar, adParamOutput, 13, "")
    objCmd.Parameters.Append parm


    objConn.Open "dsn=dsnPostman;uid=postman;pwd=postman"
    objCmd.ActiveConnection = objConn
    objCmd.Execute

   
    Dim this As String
    this = objCmd.Parameters("PARAM2").Value
    MsgBox "result " & this


But when I run the code I got the following error:
[oracle][odbc][ora]ORA-06550: line 1, column 7:
PLS-00306:wrong number or types of arguments in call to 'GENERATE_NUM'
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored

Could anyone please show me the right way to call oracle stored function from vb?

Please help.