[stared]

Any template strings are ambiguous unless there is more than one example.

For example, let's imagine that there is an instruction saying that in a config file, there should be:

PASSWORD=[password]

Let's say our password is "admin". Then it could be that:

PASSWORD=admin

PASSWORD=[admin]

or even

PASSWORD=[password]

as it is not a place to actually store the password, but to select an authentication method.

Sure, sometimes (but not always!), it is possible to deduce how to fill the pattern.

If the field has some canonical value, go with a sane default e.g. "canary.their-company.com", with a note that any other suffix works instead of "canary". Sensible defaults save us a lot of brainpower (vide https://en.wikipedia.org/wiki/Convention_over_configuration).