Config object is designed slightly different way,

It uses TransientModel, just for View declaration, however the actual data will be saved in the Params object, as you must be already aware data/record in the TransientModel can be stored only for intermediate/temporary period of time.

Hence after defining the fields, do write these two method, which acts as Saving data and Retrieving Data from the Params object.

# GETS: latest data    
def get_default_values(self, cr, uid, ids, context=None):
        
    param_obj = self.pool.get("ir.config_parameter")        
    
    res = {'XYZ_COLUMN': param_obj.get_param(cr, uid, 'XYZ_COLUMN')
         }        
    return res

similarly to store the data

# POST: saves new data    
def set_values(self, cr, uid, ids, context=None):
        
    param_obj = self.pool.get("ir.config_parameter")
        
    for record in self.browse(cr, uid, ids, context=context):
            
        param_obj.set_param(cr, uid, field, record['XYZ_COLUMN] or '0')

Note:

1. The above referenced sample code is in traditional style, as it was defined in osv.osv_memory but works well in V8, So the fundamentals stays true, change it to suite your new API style

2. no need to create a column name prefixed with 'default_', and it's better to avoid this, since these kind of prefixes serves a different purpose when used in the relevant places.

Hope this helps you.