- Create an Auto-correct rule that turns your chosen character sequence (e.g. "\t") into a regular TAB.
- Open Notepad++ (or the text editor of your choice) and type one TAB. Select it and copy it.
- Go to OneNote and click File | Options | Proofing | AutoCorrect Options.
- In the dialog, put your desired sequence of characters (e.g. \t into the "Replace:" field.
- Paste the TAB that you copied from Notepad++ into the "With:" field and click Add.
- Click OK 2 times to close the dialogs and you're done!
Now, any \t followed by a SPACE will be replaced by a regular TAB.
I found this answer on the MSFT community site.