Custom Message Box (Form) Returns Result

I've created a custom MessageBox (a Form), that has buttons Yes and No. There is a function popUp() to show the MessageBox. I would call the MsgBox in a parent as follows:
CustomMessageBox.popUp()

Now I want the message box to return a value:
Dim a as Intger = CustomMessageBox.popUp()
But this should only return once the user clicks a button in the message box.

My popup sub:
Dim thread As New Threading.Thread(AddressOf GetResult)
Public Function PopUp(Optional ByVal ErrorMessage As String = "", Optional ByVal Title As String = "", Optional ByVal ButtonType As ButtonType = ButtonType.OK, Optional ByVal MessageType As MessageType = MessageType.None)
[Code] .....
gotResult is a Boolean = false. It the user clicks a button gotResult is changed to true.
result is a integer containing the number of the button clicked.

When I run this code, the result is returned, even if the user doesn't click a button. I understand why, because the main thread of the form is not "paused" while the user has not clicked a button. How can I pause the message box itself, until a button was clicked?
I've tried this:

Public Function PopUp(Optional ByVal ErrorMessage As String = "", Optional ByVal Title As String = "", Optional ByVal ButtonType As ButtonType = ButtonType.OK, Optional ByVal MessageType As MessageType = MessageType.None)
[Code] .....

But now the whole message box is captured in a loop and the message box is like disabled, because it waits for the loop to end, so the user can't click anything.