That depends on the type and scope of the power of attorney. There are ordinary powers of attorney granted between living persons, which may be oral or written and expire when one of the parties loses their legal capacity. In addition, there are powers of attorney for healthcare and, for example, the appointment of an executor upon death, which are usually in writing. There are templates available online. A written power of attorney must be signed. The signature can be notarized.
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Power of Attorney—On Behalf of and for the Benefit of Third Parties
Template/Sample: Special Power of Attorney