Default Button in Windows Forms using .NET
This property enables you to designate a default action to occur when the user presses the ENTER key in your application. The button assigned to this property must be an IButtonControl that is on the current form or located within a container on the current form.
You can use this property to allow the user to quickly navigate a simple form by allowing them to simply press the ENTER key when they are finished instead of manually clicking the accept button with their mouse.
The accept button might not be activated if the currently selected control on the form intercepts the ENTER key and processes it. For example, a multiline text box control allows the ENTER key to be pressed when it is selected to insert a new line character in the control.
this.AcceptButton = buttonOK;
Windows forms Accept Button
For doing the same in VB.Net
Refer : http://dotnetdud.blogspot.com/2007/06/vbnet-setting-default-cancel-buttons.html
And for doing the same in VB/VBA please refer : Setting Default & Cancel Buttons in VBA/Visual Basic