1) Main Form
2) PageControl1 on MainForm
3) tabpageOwner on PageControl1
4  FrameOwner1 on tabPage
5) FrameOwnerInfo1 on FrameOwner1
6) OwnerComboBox on FrameOwnerInfo1
7) OnExit event which validates OwnerComboBox data.

Have menu selection which runs a procedure which validates the data entered
before further processing:
1) User on tabPageMain (NOT on tabpageOwner tabpage)
1) User selects Validate menu
2) OwnerComboBox has incorrect data
3) "OwnerComboBox.setFocus"  triggers an error message:
    "Cannot focus on a disabled or invisible window."

So, what Delphi command/code do I need to change the GUI from the
tabPageMain to the owner tabpage and to the OwnerComboBox component?

It needs to be "generic," because I have several situations like this in the
program.

TIA