How to use !Sub with PinPoint EmailTemplate DefaultSubstitutions?

Question

Hello, I'm trying to have template parameters passed onto a `Pinpoint` `EmailTemplate` resource as `DefaultSubstitutions` for the `HtmlPart` property.

Most attempts give me an invalid json message during the cloudformation processing (using sam deploy from cli).

```
DefaultSubstitutions: !Sub "{\"S\":\"${S}\",\"H\":\"${H}\",\"F\":\"${F}\"}"
```

ends up rejected as: DefaultSubstitution is not a valid JSON

Temporarily simplifying it to

```
DefaultSubstitutions: !Sub "{\"S\":\"${S}\"}"
```
ended up processed as:

```
"DefaultSubstitutions": {
  "Fn::Sub": "{\"S\":\"${S}\"}"
},
```

Help!
Thanks

Answer

I'm slowly coming to the conclusion that pinpoint is broken :-(
My last attempt before giving up, the "long" form:
```
      DefaultSubstitutions: >
        {
          "S": { "Fn:Sub": "${S}" },
          "H": { "Fn:Sub": "${H}" },
          "F": { "Fn:Sub": "${F}" }
        }
```

goes through with no complaints from cloudformation, but I end up with this kind of substitutions in the outgoing email:

```
{Fn:Sub="${S}"}
{Fn:Sub="${H}"}
{Fn:Sub="${F}"}
```

which is really sad :-(

Answer

So what do you want it to end up processed as?  I see https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-pinpoint-emailtemplate.html#cfn-pinpoint-emailtemplate-defaultsubstitutions describes this property as a String that's a JSON object.  What you've got above looks valid for that.

Also have you had a look at https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-ToJsonString.html?  It's interesting in that it shows JSON objects in the same format.  Also it might be useful, but maybe overkill for such a simple object.

How to use !Sub with PinPoint EmailTemplate DefaultSubstitutions?

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