The SE management world goes nuts over the terms Agile, Scrum, Kanban, and any other idea that makes it easier to get more work out of the same number of people and predict release dates. What happens when the deadlines and work intake doesn't lend itself to anything like that?
You pretend that it does! You get an estimate of how much time it takes to complete a vague description of the work, management sets a due date, and you hope for the best! You go on creating and refining stories like normal and then work out if the deadline is within the realm of possibility. Of course that doesn't happen until a ways through the project, but the sooner possible failure is detected, the more that can be done to avert it.
If completion by the date promised is not going to happen, then you have a few options to take to management:
Be wary of places with this setup because eventually some degree of #4 will be in your future. Each death march will take its mental (and possibly physical) toll on you, so best to avoid it if you can.