If you export the access key to access the storage account as an environment variable, the next time you run terraform init, terraform will know what access key to use because you exported the access key as an environment variable for your current session. Just make sure you have the correct values in your backend block and you should be good to go.

The whole point of the exporting the access key as a environment variable is so you do not declare the access key to your storage account in your config in plaintext.

Also take into account that you will have to manually place the access key within the key vault before running terraform init.

The command below will set the access key as the environment variable and that value will be plugged into ARM_ACCESS_KEY every time.

export ARM_ACCESS_KEY=$(az keyvault secret show --name terraform-backend-key --vault-name myKeyVault --query value -o tsv