You must countersign on this line of the contract.
Examples of countersign
countersign
The Prime Minister can also countersign decrees and laws passed by the President.
From the en.wikipedia.org
The relevant ministers had to countersign executive decrees for them to be binding.
From the en.wikipedia.org
Dubke remains a consultant and has authority to countersign its checks.
From the nytimes.com
The countersign has been mandatory since the revision of 1840.
From the en.wikipedia.org
The Prime Minister signs decrees establishing regulations, which the concerned ministers countersign.
From the en.wikipedia.org
White had said yesterday he wouldn't countersign.
From the bloomberg.com
I had to countersign because he was under 18.
From the morningstaronline.co.uk
The councillor had to countersign a royal decision, unless it was unconstitutional, whereby it gained legal force.
From the en.wikipedia.org
The ministers refused to obey his demand that they countersign his decision, and immediately left for Christiania.
From the en.wikipedia.org
More examples
Add one's signature to after another's to attest authenticity; "You must countersign on this line of the contract"
Password: a secret word or phrase known only to a restricted group; "he forgot the password"
Countersignature: a second confirming signature endorsing a document already signed
Countersigning means writing a second signature onto a document. For example, a contract or other official document signed by the representative of a company may be countersigned by his supervisor to verify the authority of the representative. ...
In military terminology, a countersign is a sign, word, or any other signal previously agreed upon and required to be exchanged between a sentry or guard and anybody approaching his or her post. ...
(Countersigned) Signing of a document by more than one official, e.g., a training grant program director and the institutional business official.
An exchange of a secret challenge CODEWORD and its PASSWORD reply, usually called "sign/counter-sign"; see SIGN.
A secret word or signal which must be given to a guard or sentry by someone wishing to pass; password.
A countersign consists of two words: the secret challenge and its password. The words comprising the countersign are issued from the principal headquarters of a command to aid guards in their scrutiny of persons who apply to pass. These words are disseminated only to friendly personnel.