Pattern for updating references of interface implementations

I have the following interface:

 public interface ISearchProperties {
    string CurrentUserLocation { get; set; }
    string SearchUsername { get; set; }
 }

With the following implementations:

public class BroadcastPreviewDto : ISearchProperties {
    // other properties
}
public class ProfileSearchDto : ISearchProperties {
    // other properties
}

I have the following functions:

public void PrepSearchProperties(ref ProfileSearchDto query) {
    // do a bunch of stuff to query here (only on ISearchProperties properties)
}
public void PrepSearchProperties(ref BroadCastPreviewDto query) {
    // do a bunch of same stuff to query here (only on ISearchProperties properties)
}

The problem is that this isn't very DRY - the function bodies are exactly the same thing. I tried doing this:

public void PrepSearchProperties(ref ISearchProperties query) {
    // do a bunch of stuff to query here
}

But this doesn't quite work unless I declare the original query as ISearchProperties, which strips the implementing class properties.

What pattern can I follow to DRY my code up?