For the runtimeConfig field there's special behaviour for options containing a / character. Since this is not allowed in HCL you can just use _ which is internally replaced with a / for generating the kind config. E.g. for the option api/alpha you'd name the field api_alpha and it will set it to api/alpha when creating the corresponding kind config.
Example Usage
Copy the following code into a file with the extension .tf to create a kind cluster with only default values.
Then run terraform init, terraform plan & terraform apply and follow the on screen instructions. For more details on how to influence creation of the kind resource check out the Quick Start section above.