code: @SuppressWarnings({"unused", "WeakerAccess"})
      public void addMultiValueForKey(String key, String value) {
  
          //noinspection ConstantConditions
          if (value == null || value.isEmpty()) {
              _generateEmptyMultiValueError(key);
              return;
          }
  
          addMultiValuesForKey(key, new ArrayList<>(Collections.singletonList(value)));
      }
docs: Add a unique value to a multi-value user profile property
  If the property does not exist it will be created
  <p/>
  Max 100 values, on reaching 100 cap, oldest value(s) will be removed.
  Values must be Strings and are limited to 512 characters.
  <p/>
  If the key currently contains a scalar value, the key will be promoted to a multi-value property
  with the current value cast to a string and the new value(s) added
  
  @param key   String
  @param value String