How to put data bindings in JSON field of API action?

I made a server action with the ‘api action’.
Then I want to use a data binding for an ID:

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However, in this example you can see it throws an error, which doesn’t allow me to save.
I need a way to ‘escape’ the data binding, I think…

I can get it working by putting the data binding in a string via quotes: "{{USING_THIS_CONTACT_ID}}"

But then I get an error back from the endpoint, as it expects an integer, not a string.

How can I use a data binding here?