Thanks for posting your question in the Microsoft Q&A forum.
Here's an example query that demonstrates how to achieve this:
AzureActivity
| where ResourceProvider == "MICROSOFT.STORAGE"
| where Resource == "<YourStorageAccountName>"
| where OperationName == "Microsoft.Authorization/roleAssignments/write"
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful