Add text to the end of the frame

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It would be fantastic if you can show use exactly what you are trying to do - that way we can offer suggestions and/or methods on how to proeceed.

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If it's a single story in the Frame it would be possible.

So your frame cannot be threaded to another text frame on another page

To add the text to the end of the frame you'd search for 

.+(?!\r)$

and change to

123

But it can only work on a single text page with no threaded text to another page.

If you text frames are all separate on the page - and not threaded it will add to all the text frames

Plus if you run the search more than once - you will end up with multiple inserts of 123 

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If you need it on every page - then insert it on the Parent Page in the bottom right, top right, wherever you want it and apply a Text Wrap to the text frame

This will allow all the text that interacts with the Text Frame to automatically go around it. 

It would then appear on every page at the position you select.