1. In the Azure portal, click All services, click Azure Cosmos DB, and then click the cosmos10277521 Azure Cosmos DB account.
2. In the resource menu, click Alert Rules to open the Alert rules page.

3. In the Alert rules page, click Add alert.

4. In the Add an alert rule page, specify:
Metric: Available storage
Condition: Less than
Threshold: 100,000,000 bytes

Reference:
https://docs.microsoft.com/en-us/azure/cosmos-db/monitor-accounts