If you want to attach, for example, a light to an InterpActor, you first of all need to make sure that you have a dynamic version of the actor that you wish to attach. In my example, it can be found in the Actor Browser | Lights | PointLights | PointLightMovable. Remember that some actors are static and cannot be moved, and thus cannot be attached.
Next, open up the properties of whatever actor you want to attach, and navigate to Attachment. Enter the name of the actor you want to attach to in Base. In my case, my InterpActor is called InterpActor_0, so I typed that in Base and hit Enter, and I got something similar to the following screenshot:
Argentina
Australia
Austria
Belgium
Brazil
Bulgaria
Canada
Chile
Colombia
Cyprus
Czechia
Denmark
Ecuador
Egypt
Estonia
Finland
France
Germany
Great Britain
Greece
Hungary
India
Indonesia
Ireland
Italy
Japan
Latvia
Lithuania
Luxembourg
Malaysia
Malta
Mexico
Netherlands
New Zealand
Norway
Philippines
Poland
Portugal
Romania
Russia
Singapore
Slovakia
Slovenia
South Africa
South Korea
Spain
Sweden
Switzerland
Taiwan
Thailand
Turkey
Ukraine
United States