Job DSL notifications has no support for getting URL as string/text type credential. This is supported in UI. This is important as we have api token as part of the url query string.

https://jenkinsci.github.io/job-dsl-plugin/#path/pipelineJob-notifications

https://github.com/jenkinsci/job-dsl-plugin/blob/master/job-dsl-core/src/main/groovy/javaposse/jobdsl/dsl/helpers/toplevel/NotificationContext.groovy 

There is a workaround for this. I can use below in my jenkins CasC but from UI, users can see url as paintext. I can mask url from source code but not from jenkins UI.

ex: 

notifications {
  endpoint(url = "${Credential_stringType_name}", protocol = 'HTTP', format = 'JSON') {
    event('all')
    logLines(0)
    timeout(30000)
  }
}