S9 WA states at (a):
(a) it is in writing, and signed by the testator, or by some other person in his presence and by his direction;
It does not state who the other person can be (although a beneficiary signing may throw up red flags; see Barrett v Bem  EWHC 1247 Ch)
This guidance is produced from elsewhere:
The person signing may be one of the attesting witnesses and may sign their own name. A positive and discernible communication from the testator to another person is required to amount to a valid direction to that other person to sign the Will on the testator’s behalf. The communication need not be verbal but it must be more than passive acquiescence or negative conduct, even if the testator is too weak to sign the Will or to direct another to sign it on their behalf.
There is no legislative requirement for a specific form of attestation but including one is desirable because in the absence of an attestation clause, a district judge or registrar must, before granting probate, require the due execution of the Will to be established by affidavit evidence. If a Will is duly executed, but there is no proof of execution, due execution may be presumed.
An attestation clause I have for this is:
SIGNED by the witness WITNESS in the name of TESTATOR on the instructions of TESTATOR who being able to read but unable physically to write his signature after TESTATOR confirmed he knew and approved the contents of the Will as and for his last Will in our presence and then by us in his