Hi,

Insert the following code into the action triggered by your button click event:

return {

            'name': 'New',

            'type': 'ir.actions.act_window',

            'view_type': 'form',

            'view_mode': 'form',

            'res_model': 'your.model',

            'res_id': self.id,

            'context': {'default_module_id': self.id},

            'target': 'new'

        }

Make sure to replace 'your.model' and 'default_module_id' with the actual name of your model and the field you want to set a default value for, respectively.

Hope it helps