key:

Additional fields:
  Get id captcha
  2 words
  cAse sensitive
  mathematical operation
  minimum length
  maximum length
  1 - russian 2 - english
  1 - only numerals 2 - only letters 3 - either numeric or letters
  soft_id
  id_constructor
  textinstructions
  textcaptcha


  recaptcha
  can_no_answer
  recaptchacols
  recaptcharows
Add additional flags in your code:
TAG POS=1 TYPE=INPUT:CHECKBOX FORM=ACTION:http://rucaptcha.com/in.php ATTR=NAME:regsense CONTENT=1

for min_len / max_len:

TAG POS=1 TYPE=INPUT:TEXT FORM=ACTION:http://rucaptcha.com/in.php ATTR=NAME:max_len CONTENT=15