aws cloudformation use Fn::Join in a list
<p>I've a cloudformation template that uses custom resource backed by lambda function. One of the parameters of the lambda function is a list of strings. I have only one item to pass in the list and would like to use Fn:Join to concatenate create the string. However, using Fn::Join gives error as it leads to invalid json. Any inputs are appreciated.</p>

<p>"Subscriptions": [ "Fn::Join": [":", ["a", "b", "c"]]]</p>

<blockquote>
  <p>A client error (ValidationError) occurred when calling the CreateStack
  operation : Template format error: JSON not well-formed.</p>
</blockquote>

<p><strong>Cloudformation snippet:-</strong></p>

<pre><code>  "Resources": {
"MyCustomRes": {
      "Type": "Custom::CustomResource",
      "Properties": {
        "ServiceToken": { "Fn::Join": [ "", [
                                        "arn:aws:lambda:",
                                        { "Ref": "AWS::Region" },
                                        ":",
                                        { "Ref": "AWS::AccountId" },
                                        ":function:LambdaFn"
                                      ] ] },
        "Version": 1,
        "ResourceName": { "Ref": "ResourceName" },
        "Subscriptions"       : [ "Fn::Join": [ "", [
                                        "arn:aws:sns:",
                                        { "Ref": "AWS::Region" },
                                        ":",
                                        { "Ref": "AWS::AccountId" },
                                        ":Topic1"
                                      ] ] ]
    }
}     },
</code></pre>