Permanently highlight all occurrences of text in Notepad++?

Just use the mark feature.

1. Open the Search & Replace dialog and go to the Mark tab.

2. Enter your search term and click Mark All.

   

3. All occurrences are now permanently marked:

   

To remove the marks, simply open the dialog again and click Clear all marks.

Alternatively, while you have the word selected, you can use the Mark functionality in the Search menu:

This also allows you to mark all occurrences of a given string in the document (and you can even have multiple markers going on simultaneously).

Please note that both approaches work perfectly fine with partial words. In fact, multiple markers can apply to the same partial string. Here I've used a marker for port and one for mporta:

Select your text > Right click on the text area > Choose Style Token from context menu > Apply the style you need. Done.