You can directly edit the user dictionary file on MacOS by doing the following:
Open the Finder on the Mac if you have not done so already. Hit Command+Shift+G to open the Go To Folder window and enter the following path (you need to do this because the Library is usually hidden): ~/Library/Spelling/LocalDictionary
Open the LocalDictionary file in your text editor of choice, whether that is TextEdit, nano, BBEdit, or other (if you use a word processor make sure you save as plain text).
Type the word(s) you wish to add to the user dictionary, with each new word on a separate line.
Save the LocalDictionary file.
You may need to quit and relaunch some apps to have them use the changes you have made to the dictionary.
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