How to write the certificate that an individual entrusts a company to pay social security?

Not necessary, you can apply for a certificate of entrustment. When entrusting an individual to issue relevant certificates of the insured, he/she shall fill in the social insurance enrollment inquiry registration form, and provide the original and photocopy of the client's ID card, the original and photocopy of the trustee's ID card, and the power of attorney (specifying the entrusted matters). In fact, it's best to go in person, and it's still more troublesome to entrust.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.