Request agreement for a project objective that you can execute using a realistic plan. Following agreement, you can use the plan for project baseline setting.
If a realistic analysis of your best project plan leads to recognition that the project is a bad idea, take it no further. Canceling doomed projects early is better for everyone.
If your presentation fails to convince your sponsor or falls on deaf ears, you might be forced into an infeasible project. If so, document the situation for future reference, and then see if you can find a way out of the project.