Copy Activity addition column expression

Hi @Community

Totally new to ADF and trying to learn from past 1 week. Surprisingly I felt it is more tricky'er than other Azure services.

All I'm trying to do is pretty simple. Copy data from a CSV File stored in Azure blob storage in to Dataverse (previously known as CDS). I just need to replace 1 source column text before sink. I have tried to use Expression (@replace) and it turns out it is not possible to refer the Source column in the expressions. Is that true? Did I missed any obvious step? I have googled alot and I couldnt able to find proper results. Any help?

Thank you

-Srikanth