How do I query for a partition keys that contain a specific substring in dynamoDb?

I have a partitionKey that is made up 2 strings for e.g. userId:UserName. For e.g 1234:John, 4567:Mark etc. I want to query for all the records that match the substring defined by UserName for e.g. Find all the records that contain "Mark" in the partition key. How do I do this using DynamoDb APIs in Java?