I understand that 圏点【けんてん】 are not very often used in Japanese writing as much as italics are used in English, but how do you add 圏点【けんてん】 using an English keyboard?
2 Answers
Probably not what you were asking, but as an additional note, you can add 圏点 on this site using the furigana syntax:
[圏点]{﹅﹅}
turns into
[圏点]{﹅﹅}
And one can get that character by kanji completing けんてん in most IDEs.
Note that you do need the [] around the thing you are trying to put furigana on.
I don't know why exactly you want to use a keyboard to add them, because emphasis marks, like italics in English, are not part of the text but of the formatting/typesetting.
However, common Office Software supports adding emphasis marks through their text formatting tools:
Microsoft Word
You can add emphasis marks through the Format > Font
dialog. (I'm sorry for posting a screenshot in a German language version of Office, but it's the only one I had available at the moment)
LibreOffice Writer
You can add emphasis marks through the Format > Character
dialog. You have to enable East Asian Language support through the options first if you use the English language version.