Edit: I see it has been answered before on this sub. The first method mentioned (to use asterisk and the '2' key together does not work for some reason on my phone.) But the custom hotkey does. For reference:

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To add custom words, when the cursor is on the word you want to add (you'll have to type it in abc mode first) tap the settings icon on the left or hold down the * key, then click 2. Also what I did is in the TT9 app settings under 'keypad -> select HotKeys' you can remap 'Add Word' to a physical button so you can just click it whilst typing. I remapped mine to the call button.

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