I would add the values to the combobox as follows:

thisform.combobox.AddListItem("My First Value", 100)
thisform.combobox.AddListItem("My Second Value", 200)
thisform.combobox.AddListItem("My Third Value", 300)

The above displays a value to the user (the first parameter) and the second parameter is the ListItemId. In your case assign the numeric value as the ListItemId for each item added. To get the value the user has selected, you now use:

lnUserSelectedValue = thisform.combobox.ListItemId