when user clicks the combobox want to display popup message using csharp or vb.net

Question

 please see Attach File.


in run mode when user CLICKS THE COMBOBOX1  Message will POPUP using vb.net or Csharp.
in the below user clicks the  COMBOBOX1  Message will POPUP.

 
Net Payable Days  txt_days  COMBOBOX1.

send the code.