Custom attributes that can be found in the alert defitnions cannot be passed properly by Webhook channels or message templates by setting following convention, for instance:
Custom 9:${custom_14(name)}
In order to pass custom definiton properly go to webhook or message template of your choice and click "Add Variables". Then search for the fields that can be found next to the custom values and in the brackets, for instance having custom atribiutes like below:
"custom_17(category)": "System",
"custom_13(SourceProduct)": "APM",
Refer to the "category" or "SourceProduct" and syntax with custom binding will be added automatically as below: