Hi,

You can use the following code for creating a related user for the employee during his plan launching.

class HrPlanWizard(models.TransientModel):
    _inherit = 'hr.plan.wizard'

    def action_launch(self):
        user_id = self.env["res.users"].create(
            {"login": self.employee_id.work_email, "name": self.employee_id.name}
        )
        self.employee_id.update({'user_id': user_id.id})
        return super(HrPlanWizard, self).action_launch()

In the above code, we are creating user with its require fields login and name, you can update the fields which you require in that dictionary.

Hope it helps