I don't know about the input mask properties but here's another solution;
The KeyPress event "object_KeyPress (KeyAscii as integer)" will trap the ASCII code of all new characters as they are entered into the text box/list box/ label, whatever. Set up an If...else loop checking to see if the KeyAscii value corresponds to the ASCII code for the comma (44 on my system). If this is true, add a space to the end of the full string.