I am using a Tab control in my GUI app and I know how to add and remove tabs from it (TCM_INSERTITEM, etc.) but I cannot figure out how to add other controls to a tab like B
If you are talking about adding controls to the body of the tab control: You don't.
What you do is create a child styled dialog control for each tab, which will have the controls for that tab on it. As each tab is selected, you handle the notification from the tab control to create and show the appropriate page control. The page dialogs are normally parented to the parent of the tab control, not the tab control itself.