This details how to add keywords using GeoSetter but applies to all other types of IPTC tags.
Open GeoSetter and browse to your folder of images.
Highlight the desired image by a single-click. For multiple images, select by holding down the Ctrl key.
Under Images, click on Edit Data or Ctrl+E. Or you can use the Edit button in the toolbar.
Click on the Categories/Keywords tab.
Under Keywords, type your keyword and click Enter. Repeat for each keyword. And then click OK at the bottom of this window.
Add Keywords to Multiple Images
When selecting and adding keywords to multiple images, you have to take the extra step of clicking on Set Current Values For All Selected Images at the bottom of the Categories/Keywords window.
Set Current Values For All Selected Images
Note that all selected photos on the left will now be in bold.
In the Thumbnail browser, your changed photo(s) will now be highlighted in purple.
Click on the Images menu and click Save Changes or Save Changes of Selected Images. Or use the hotkey combination.
As you add keywords, they are automatically saved to a drop-down list for future use.
Edit Most Recently Used Items
If you click the three dots next to the arrow, you can type in a list of keywords or paste them in from another source. Of course, you can also save them from this location.
Expand this process for all other types of tags such as copyright, source, contacts, captions, etc.