Choose the predictor time series as the shifted series

I am interested in the CausalImpact to analyse the impact of an event on a time series.

Now, it the serious challenge to find a proper Control Series, I think. But unfortunately in my case this might be a problem. So I came to the following idea.

What about using the shifted time series for your response? If I have click rates for the last 3 years and what to analyse the impact of my event, I think I could use the same Series shifted by let's say 1 year. This underlies the assumption that over a year for eg a specific month my Response behaves similar to last years'month. So I can use the shifted series to see if the event has an impact for this year's month?

This seems a very easy idea, but is not mentioned especially in this nice video.

Maybe I oversee some statistical consequences?

Any one who has an idea?