Here is an example based on the contract entity out of the box.
I have placed a tab at the top of the form, above 'General' with visible by default turned off.
1. I select a related page from the navigator
This opens the related page.
2. I select the 'Contract' at the top of the ribbon bar
This re-opens the main form.
The hidden tab is now visible, and furthermore the text cursor is the div with the id tdAreas, and that div appears to be in edit mode. I can actually type into that div, although nothing appears on the screen (the text caret moves).
