How to calculate the "secret" field of an activation operation?

Just to clarify the issue: the main reason for the question was that I was trying to add to my Java library, TezosJ, a feature to automatically create new Tezos addresses, because a client I am working with needs this functionallity.

By "create new Tezos addresses" I mean creating them, activating them and revealing them, in a single operation, so it becomes trivial to the end-user.

Occures that "activation" is an operation intended for those addresses from fundraiser participants (or faucets) and needs a "secret". So I had two options: Get the algorithm to create the secret (thanks Arthur, for kindly answering) or to activate an address by sending some tez to it (thanks, Klassare for the help).

What I finally did was to create a method on TezosJ called "activate", that has two different behaviours: with or without the "secret" parameter. If the parameter is present, then the method try to activate the address as it was from fundraiser or generated by a faucet, like this:

     transaction.put("kind", "activate_account");
     transaction.put("pkh", addressToActivate);
     transaction.put("secret", secret);

Otherwise, in the lack of the "secret" parameter, it assumes the proceedings are different from fundraiser/faucet and then activates the account by sending some tez to it:

sendTransaction(from, addressToActivate, amount, fee, gasLimit, storageLimit...);

Of course, by this methodology, an already known account should call the method to activate the new created account and will be charged for the operation.

Important to remember that, the main goal of TezosJ library is to always make easier for developers to build for Tezos blockchain, so whatever I can do to make things simpler, the better.

The end result for this was that, now the client I am working with can seamlessly send a JSON wrapped command ("createNewAddress") to a middleware I wrote (called Quarterback) and it interacts with Tezos blockchain by using TezosJ library. Quarterback acts like a new layer on top of TezosJ and free the client from even knowing how to code in Java. With this higher level layer it's possible to send commands to an FA1.2 smart contract without any programming. And it is working great!

Quarterback works integrated to AWS SQS (Amazon Queues). Amazon SQS is very popular between developers worldwide and now they can seamlessly integrate existing systems with Tezos smart contracts by sending JSON wrapped commands.

This is the script used to generate faucet blinded accounts.

https://gitlab.com/tezos/tezos/-/blob/master/scripts/create_genesis/create_genesis_info.py#L75