quoting from: https://docs.microsoft.com/en-us/azure/kusto/query/joinoperator#cross-join
Kusto doesn't natively provide a cross-join flavor (i.e., you can't mark the operator with kind=cross). It isn't difficult to simulate this, however, by coming up with a dummy key:
X | extend dummy=1 | join kind=inner (Y | extend dummy=1) on dummy
