So if we had an [IMDB Series ID] field, a link from a file could be set up using Manage Links, as below.
<Link version="1.2">
<Item Name="Name">IMDB</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">https:////www.imdb.com//</Item>
<Item Name="Search URL">IfElse(!IsEmpty([IMDB ID],), https:////www.imdb.com//title//[IMDB ID]//, !IsEmpty([IMDB Series ID],), https:////www.imdb.com//title//[IMDB Series ID]//,1,https:////www.imdb.com//)</Item>
<Item Name="Filter"/>
</Link>
As that field doesn't exist yet, I put the IMDB Series ID into the [Comment] field, using the setup below.
<Link version="1.2">
<Item Name="Name">IMDB</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">https:////www.imdb.com//</Item>
<Item Name="Search URL">IfElse(!IsEmpty([IMDB ID],), https:////www.imdb.com//title//[IMDB ID]//, !IsEmpty([Comment],), https:////www.imdb.com//title//[Comment]//,1,https:////www.imdb.com//)</Item>
<Item Name="Filter"/>
</Link>
That worked fine.
When the [IMDB ID] field contained a value, it open the IMDB wep page for the Movie or Episode.
When the [IMDB] field was empty, but the [Comment] field had a value, it open the IMDB wep page for the TV Series. (Assuming I had a IMDB Series ID in the [Comment] field for the selected file, of course.)
When neither the [IMDB] or [Comment] fields had a value, it opened the main IMDB page.
So having an [IMDB Series ID] would be quite useful, when combined with a custom Link in MC.