This should work:
typedefof<System.IEnumerable<_>>
EDIT
As Tomas notes, there’s nothing special about the _ wildcard here; F# infers that the type obj is the most general applicable type in this context, so this is the same as using typedefof<System.IEnumerable<obj>>. In some cases the way this works can be a bit of a hindrance, though. For instance, if you define an interface type I<'a when 'a :> I<'a>> = interface end, then you can’t use typedefof<I<_>>, because I<obj> doesn’t satisfy the generic constraint and F# can’t infer another more appropriate type. This can happen even without recursive constraints (e.g. type I<'a when 'a : struct and 'a :> System.ICloneable> = interface end. This is in contrast to C#’s approach, which works perfectly fine in the analogous cases.
As to your code itself, I think you’ll want to make some other changes, too, such as ensuring that the interface is generic before calling GetGenericTypeDefinition. Here’s how I’d write the test function:
(fun t -> t.IsGenericType && (t.GetGenericTypeDefinition() = typedefof<_ seq>))