Just to confirm, and maybe SB should have mentioned it.
I think old age is getting to him, and sometimes he forgets crucial information.
If there is no IMDb page at all for the series/movie, then you click on "search" on the upload page, scroll down to the bottom, and add movie name and year to the OS database manually. The ID number given in the end is the number you put where the IMDb ID would normally go. Hope that makes sense and helps.
But please do remember, when that series/movie has an IMDb ID available, to change it on those subs with the IMDb ID to replace the OS ID number that you created.
When we can upload to https://www.opensubtitles.com/en/home
, hopefully that site will have TMDB input, too.