It's not currently possible to accredit powers of attorney to third parties for your Tomorrow account. In the name of being honest, we'll admit that this isn't very high up on our to-do list as it's not frequently requested.
However, we understand it is (justifiably) important to you, so here's our tip:
Search for a standard power of attorney form online and fill it out together with the person you want to authorise. Then, you can keep the completed form in a safe place and, should the worst happen, the power of attorney form can be presented to us and together we'll find a reasonable solution to settle your affairs for you.