To create a subaddress, you need the wallet private keys. Assuming you have these, you can create them in javascript using this package, as an example.

You cannot do this only having the public key, you need the private keys.

To create a subaddress, you need the wallet private keys. Assuming you have these, you can create them in javascript using this package, as an example.

You cannot do this having only the public key, you need both the private view key and public spend key.