Hi Zied dams,
tax_totals_json is a computed field and the method that computes the value is "_compute_tax_totals_json". You need make the following changes to add field inside tax_totals_json field.

• You need to change this method and append your key, value pair in the tax_total_json. here is an example: 
   
   
   import json
   
  class AccountMove(models.Model):
      _inherit = "account.move"
      
      your_field = ....
      
      def _compute_tax_totals_json(self):
          super(SaleOrder, self)._compute_tax_totals_json()
          for rec in self:
              tax_totals_json_dict = json.loads(rec.tax_totals_json)
              tax_totals_json_dict.update(
                  {'your_field': rec.your_field})
              rec.tax_totals_json = json.dumps(tax_totals_json_dict)
  
  

• Now you have to inherit the "account.TaxtotalsField" template  in extension mode.this xml file should under  "static/src/xml/"
  

• Then using xpath add your desired field in the position you prefer. here is an example for steps 2,3:
   
  
  
  Sorry unable to incule the example, Dosen't let me add it here
  
  
   

• Lastly, you have to include the xml in the menifest as 'asstes'.  Add the following key, values to your manifest dict```
      'assets': {
          'web.assets_qweb': [
              'you_moduel_name/static/src/xml/your_xml_file_name.xml',
          ],
      },
  ``` 

Hope this helps.