how can i auto populate in smartsheets

This may not be possible. It depends if your assignee column is in a contact column or not. Contact columns cannot contain formulas, therefore they are not automatable. If you just need a name reference there, then you could make your asignee column a text/number type field and use a formula similar to the following: =IF([Campaign Type]23 = "Video", "User's Name")

However, you won't be able to send any emails, updates, or data based on that column. So there are drawbacks.