You can form a partition key by concatenating multiple property values into a single artificial partitionKey property. These keys are referred to as synthetic keys.
Another possible strategy to distribute the workload more evenly is to append a random number at the end of the partition key value. When you distribute items in this way, you can perform parallel write operations across partitions.
Note: It's the best practice to have a partition key with many distinct values, such as hundreds or thousands.
The goal is to distribute your data and workload evenly across the items associated with these partition key values. If such a property doesn't exist in your data, you can construct a synthetic partition key.
References:
https://docs.microsoft.com/en-us/azure/cosmos-db/synthetic-partition-keys