Can anyone suggest a way of making the AWS DynamoDB TTL declaration conditional?

resource "aws_dynamodb_table" "this" {
  name         = join("-", compact([, var.purpose]))
  billing_mode = "PAY_PER_REQUEST"
  hash_key     = var.hash_key
  range_key    = var.range_key
  ttl {
    enabled = 
    attribute_name = 

Attempts I’ve made often end up stating:

│ Error: Unsupported block type

│ on modules/services/dynamodb/ line 24, in resource “aws_dynamodb_table” “this”:
│ 24: ttl {

│ Blocks of type “ttl” are not expected here.

I have several DynamoDB’s tables to create, but only a couple of tables will need the TTL setting declared. Hence why I am looking to make the TTL block conditional.