From
https://osm-boundaries.com/
The db-value isn't visible in it's raw format. But as long as you don't change the DB-selector below the map it should be set to osm20200810 as the current ones are. But if you change the db-selector below the map to for example "2020-08-03" you should change the value to osm20200803.
The values after osmIds= are the IDs of the polygons you have selected. For example; If we make up a fake country that should consist of polygons for Albania and Afghanistan we would select those two in the tree view at the site. When we have the polygons we need we right click the tree and select the list item in the context menu (Copy all selected IDs). Now the list of IDs is copied and we can just paste it into the wiki.
(a fake country consisting of Albania and Afghanistan is of course a very unrealistic example)
A more realistic example that's fairly known to most is USA. United States includes Puerto Rico (and some others) that according to HQ (and some definitions) aren't a part of USA. Therefore we can't select the outer level called "United States" because it would include Puerto Rico as well. Instead we select every single state that USA has.
Example screenshot of USA:
https://1447.se/shutter/screenshot_20200909_20%3A47%3A21_9177.png
I might have selected too many or too few polygons for that example, just made it as a proof of concept.
Normally all data should exist and work with the latest database (2020-08-10), but I know for example that Thailand is broken in that data-set. However, the database 2020-08-03 has temporary become disabled, it should show up in 1-2 days again.