'Creates a new cosigner resource.
:param xpubkey1: a bip32 extended public key (customarily the hot key)
:param xpubkey2: a bip32 extended public key (customarily the cold key)
:param email: a contact email
:param billing_plan: the billing plan for the cosigner'

def create(self, xpubkey1, xpubkey2, email, billing_plan='electrum-per-tx-otp'):

    payload = {'email': email, 'xpubkey1': xpubkey1, 'xpubkey2': xpubkey2, 'billing_plan': billing_plan}
    return self.send_request('post', 'cosigner', payload)
