Adding MsgBox to Userform

I have 2 userforms feeding data to the same rows in a worksheet. Userform1 user inserts the batch number in the combobox and other data into the worksheet and clicks the OptionButton to notify Userform2 that he is required to enter the serial number. If Userform2 clicks on the matching batch number in the dropdown combobox, and Userform1 has clicked on the OptionButton indicating a serial number is required, then I would like a message box to pop up on Userform2 to notify the user that he needs to enter a serial number in the serial number textbox. The message box would only need to have an 'Ok' button to close. I have added an attachment for clarification.

Hello BuckChow,

After you have tested your conditions for the MessageBox to display your message, this all you need to do...

MsgBox "Your message goes in the quotes"

That's it. The default button is "OK". Your macro will stop running until the user responds to the Message Box.

Sincerely,
Leith Ross