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?
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.