Custom Property of Data Types NULL value

I know I can use the default basic “Parameter” dataset to define a param for a Data Type Tag. But there is this Custom section where it’s supposed that you can create a custom property. I still don’t get it how to make it work because I’m always reading a null value.
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First I can’t see it here


but I tried
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When creating the instance
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and
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I know there’s a simple way to use it but I don’t know how. Any help?