code: def add_dbxref(self, feature_id, db, accession, organism=None, sequence=None):
          
          data = {
              'features': [
                  {
                      'uniquename': feature_id,
                      'dbxrefs': [
                          {
                              'db': db,
                              'accession': accession,
                          }
                      ]
                  }
              ]
          }
          data = self._update_data(data, organism, sequence)
          return self.post('addDbxref', data)
docs: Add a dbxref to a feature
  
          :type feature_id: str
          :param feature_id: Feature UUID
  
          :type db: str
          :param db: DB Name (e.g. PMID)
  
          :type accession: str
          :param accession: Accession Value
  
          :type organism: str
          :param organism: Organism Common Name
  
          :type sequence: str
          :param sequence: Sequence Name
  
          This seems to show two attributes being added, but it behaves like those two are one.
  
          :rtype: dict
          :return: A standard apollo feature dictionary ({"features": [{...}]})