Message Link is the URL the user will be redirected to when they click the Web Push notification. You can provide it in two types:
Custom link
The custom link is the static URL you enter manually.
Dynamic link
When you select the dynamic link, you can use default or custom attributes, which should be pre-defined in custom user attributes. You can also define a fallback to ensure that your Web Push messages are not disrupted if the URL source value is empty.
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The Fallback URL field will automatically display the value set in the Web Push Settings on the screen. If you leave the input field empty, the fallback value will still be used.