Hi,

Many2one relation returns records of related model. Many2one field store record id of that related model. For reference check this link​   

You can use default method to load default value in many2one field 

field = fields.Many2one('related_model', string="", default=_default_value, required=True)

@api.model
def _default_value(self):
    return self.env['related_model'].search([], limit=1)

The value in "x" is the record that you have selected.
It stores the 'id' of that referencing record. So directly you cannot store the "value" to that, but by setting the id of the record will set the value. It is Integer value that you can set in "x".