Hi, you can do it like that, Like DonManFred said:
B4X:
Dim count As Int = 0
For Each p As TabPage In tp.Tabs
If count = 1 Then
Dim btn As Button
btn.Initialize("")
btn.Text = $"Button added to page 2!"$
p.Content.AddNode(btn, 50, 50, 200, 100)
End If
count = count + 1
Next
tp.SelectedIndex = 1
Here that creates a button on the second page (page index 1, because the first page is index 0). So the pane of the TabPage is accessed via "p.Content".
By the way, I don't know why I went as far as using JavaObject in the first example, but to access the tabs, just use: