Is there a way to automatically add periods at the end of a sentence or paragraph when ENTER key is pressed?
For a single-sentence paragraph, you can use autonumbering to add any character at the end of the paragraph … but the character is then positioned at the right-hand margin, which probably isn't what you want :-}
What you could do:
Create a new paragraph format, e.g. "period" with autonumbering ".", at the start of the paragraph.
In the Pagination tab of the original paragraph with the text set Run-In Head. In the Basic tab set "period" as Next Paragraph Tag.
When you now press ENTER twice, you get your period.
You could also create an EventScript with ExtendScript. That one could also check if you already have a "." at the end of the paragraph and only add a "real" period (not just a generated numbering), if that is not the case.
(I'm just wondering what the use case for this request it?)