Opt-In without privatekey

Is there a way to opt-in without privateKey?
I don’t want to expose it for certain reason. (don’t want to use desktop-wallet)

I’m using nis(nem.core daemon) version 0.6.97 and send transaction, sign with privatekey via daemon
Is there a way to use nem core daemon? like using api

symbol-migration-page mentions that
Advanced users can use a series of on-chain transactions which can be prepared manually and issued on the NEM NIS1 blockchain programmatically. This route is for very specific use cases. If you don’t know why it might be used, it is not for you.

What is it about?

Sorry for the question

At this moment there is no tool that is preparing transaction just to announce it.
If this is only opt-in without namespaces, vrf keys, multisig then message can prepared because it’s simple one (contains Symbol public key message)