You don't "have" to put anything in the field, but it may not match correctly if you do.
As far as values you can use, both "comicvine:xxxxx" and "comicvine-volume:xxxxx" are valid values for it it search on. As an example, let's look at say, Batman V3. If you search for Batman and bring up the Volume 3 ongoing series (the current one), you'll get this link:
https://comicvine.gamespot.com/batman/4050-91273/
The last number sequence is the comicvine-volume value for the series. So if you wanted to tag one or more Batman V3 comics, you'd tag them as comicvine-volume:91273. As long as you have something in the title to denote what issue the comic is it should identify fine from there.
Suppose then we go to the most current issue of Batman V3. That brings you to link
https://comicvine.gamespot.com/batma...6/4000-741783/
Same idea. If you want to tag the issue directly, you can just do "comicvine:741783" And it'll identify it as that exact issue. I generally use this for special issues where there's only one issue since it's a bit faster.
Unless you have a high level of accuracy in your titles, I would suggest reviewing each issue before saving, because unless you pre-tag everything with an identifier it will regularly miss the correct issue.