Try with this snippet:
- (id)initWithDictionary:(NSDictionary *)dictionary error:(NSError *__autoreleasing *)error {
NSDictionary *hookedValue = [[NSDictionary alloc] initWithObjectsAndKeys:
@"defaultValue1", @"defaultKey1",
@"defaultValue2", @"defaultKey2",
@"defaultValue3", @"defaultKey3",
nil];
dictionary = [hookedValue mtl_dictionaryByAddingEntriesFromDictionary:dictionary];
return [super initWithDictionary:dictionary error:error];
}