I am typing Greek documents and I do not know which key to stroke in order to put the symbol of accent above the letter that needs to be accented in any given word

I am using LibreOffice Writer component.
The document language is Greek from an online dictionary.
The language of the interface and that of the operating system is English (US).
The operating system is Ubuntu, the latest version.

So far I am typing the document in Greek without accent. Then I am running a spell check, during which one by one (manually) I pick the correct option of the word I want, which then comes with the accent of course. It would be very handy if I can add the accent while typing the letters / words.

Here is an example of my name when I type it Πηνελοπη. And here is how the word should look like if properly typed (as pasted after corrected manually from the dictionary) Πηνελόπη

Various answers in this question.