Can I apply to become a legal representative for my husband to do his taxes?
You can apply to be your husband's legal representative for tax purposes, but it is not required that you do this to use TurboTax.
This CRA page tells you how to register for Represent a Client:
https://www.canada.ca/en/revenue-agency/services/e-services/represent-a-client/about-represent-a-client.html#q3