I am writing a software user manual so the topic is about possible situations which its users may come across.
Here is an example sentence below:
Enter the window ID which contains an image.
This is a perfectly acceptable sentence. The fact that the window is a hypothetical future window doesn't affect the fact that it is a single, definite window within your discourse, and therefore needs the definite article.
The sentence is fine as is, but a little ambiguous.
It might help if you were more explicit. For example:
Or, if "window ID" is a phrase defined elsewhere in the manual