Thank you for posting query in Microsoft Q&A Platform.
As per my understanding, you want to write expression for filename. But not able to use column names inside expression. Please correct me If I am wrong.
While writing expressions for file names we cannot use column names there. We can use parameters there. So we should have our desired values in to parameters and use them in to file names.
Please check below screen shots which gives better idea.
Hence, please consider having your year and month values in to dataflow parameters.
Also, you may consider using cache lookups. Please check this video to understand cache lookups and cache sinks and see if that helps. Thank you.
Hope this helps. Please let us know if any further queries.
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