Type the word. When you get a red underline, right-click it and select "Add to dictionary". Do this for each word and they won't be highlighted as incorrect ever again ;)
The only disadvantage is that these words would be highlighted once you open the document on a different PC. Unfortunately, there isn't much you can do about this except add those words to the dictionary on those computers too.
You can also add the words in one PC, export the dictionary, and then import that dictionary in all other PCs... (https://www.extendoffice.com/documents/word/870-word-import-and-export-custom-dictionaries.html ) Here's how:
Step 1: On the source computer, copy the custom dictionaries in
C:\Users\\AppData\Roaming\Microsoft\UProof to an
intermediate location that the destination computer can access.
Step 2: On the destination computer, click File > Options, select the
Proofing and click the Custom Dictionaries (In Word 2007, click the
Microsoft Office Button, and then click Word Options).
Step 3: Click Add to import the custom dictionaries from intermediate
Note: If you have several dictionaries to export, you have to repeat step3 several times. Some examples of intermediate locations that the destination computer can access include an e-mail account, a floppy disk, or a CD