Habe ich in Google gefunden? Kann das sein?

...nie gehört - Terminblocker.

Terminblocker
means in German "I /reserve a time slot for a non-confirmed meeting, just as an option".

Is in English a similar word for this optional time slot?

The mentioned words in the forum are all names of Office Supplies, these have nothing to do with the word "Terminblocker"

A "Terminblock" is a paper schedule, but "Terminblocker" is something different.
German can be as complicated as English is :-)

"save the date" vielleicht? Ist aber nur ein Vorschlag!

according to a native speaker this would be the proper translation