As far as I know, there's 2 ways to go about this (I think they're functionally identical):
1. Use the CB_SELECTSTRING message
2. Use the CB_FINDSTRING/CB_FINDSTRINGEXACT + CB_SETCUSEL pair.
SendMessage(comboHwnd, CB_SELECTSTRING, 0, (LPARAM)buffer);
or
index = SendMessage(comboHwnd, CB_FINDSTRING, -1, (LPARAM)buffer);
if (index != CB_ERR)
SendMessage(comboHwnd, CB_SETCURSEL, index, 0);