Hi  Simon ,

You can use related if the field provides the value of a sub-field on the current record

employee_id = fields.Many2one('hr.employee', string="Employee", default=lambda self: self.env.user.employee_id.id,help='Name of the employee for whom the request is creating')
department_id = fields.Many2one('hr.department', string="Department", related='employee_id.department_id',
                                help='Department of the employee')

or you can use 'default'

your_field_name = fields.Many2one(
    comodel_name='your.model', default=lambda self:
    self.env['your.model'].search([], limit=1))

In XML 

<field name="your_field" force_save="1" readonly="1"/>