You can use Business Rules triggering After an operation to display the required message in the Execution Log. However, the execution log will contain all the Business Rules triggered by the operation and performed actions, not only the one with the message. Unfortunately, there is currently no other possibility. However, we have the feature in our roadmap.
If the approach meets your needs, the Business Rule should look like the following:
In the run PowerShell script action, use the below script, where <TEXT> should be replaced with the message you need to output.