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I’m not a cosmosdb expert, but from looking at analogous ARM templates, it appears that you would specify indexing policies in the mongodb collection resource: <https://www.pulumi.com/docs/reference/pkg/azure-native/documentdb/mongodbresourcemongodbcollection>
And specifically in the `resource` property <https://www.pulumi.com/docs/reference/pkg/azure-native/documentdb/mongodbresourcemongodbcollection/#mongodbcollectionresource>

Thanks Mitch. It actually turns out that you can't because the underlying REST API doesn't support it. The index keys are just strings and so cannot individually indicate ascending/descending