How to pass type to function argument in Go

You can’t. You can only pass a value, and CustomStruct is not a value but a type. Using a type identifier is a compile-time error.

Usually when a “type” is to be passed, you pass a reflect.Type value which describes the type. This is what you “create” inside your getTypeName(), but then the getTypeName() will have little left to do:

func getTypeName(t reflect.Type) string {
    return t.Name()
}

// Calling it:
getTypeName(reflect.TypeOf(CustomStruct{}))

(Also don’t forget that this returns an empty string for anonymous types such as []int.)

Another way is to pass a “typed” nil pointer value as you did, but again, you can just as well use a typed nil value to create the reflect.Type too, without creating a value of the type in question, like this:

t := reflect.TypeOf((*CustomStruct)(nil)).Elem()
fmt.Println(t.Name()) // Prints CustomStruct