Do you mean to write it on the check you send in the mail? No.
Any type of Efiled payment would need to have your SSN associated with it.

(Side bar....my husband asks me this same question EVERY TIME he has to make one of the estimated payments.  We've been married for 32 years in May.  You do the math   ):joy:

The CA payment vouchers tells you to include the SSN on the check, so if clients ask me if they should include it I generally tell them yes.

Ive had so many people say "But I don't want them to see the SSN on the check", ummm, only the tax agency and the bank are going to see it and they both already have your SSN.....if it gets lost in the mail, the payment voucher with your SSN is right there with it, so its hardly a secret.

That's why I just tell John "sure".

:joy: