CPMSifDlg::EncodeAndSend() method is declared as non-static and thus it must be called using an object of CPMSifDlg. e.g.
CPMSifDlg obj;
return obj.EncodeAndSend(firstName, lastName, roomNumber, userId, userFirstName, userLastName);
If EncodeAndSend doesn't use/relate any specifics of an object (i.e. this) but general for the class CPMSifDlg then declare it as static:
class CPMSifDlg {
...
static int EncodeAndSend(...);
^^^^^^
};