Well, i figured out how to make it works:
//get folder
ns1__GetFolderType *gftRoot = new ns1__GetFolderType();
gftRoot->FolderIds = new ns3__NonEmptyArrayOfBaseFolderIdsType();
gftRoot->FolderIds->__size_NonEmptyArrayOfBaseFolderIdsType = 1; //there we specify how many folders elements in FolderIds array
gftRoot->FolderIds->__union_NonEmptyArrayOfBaseFolderIdsType = new __ns3__union_NonEmptyArrayOfBaseFolderIdsType(); //create one element of array
gftRoot->FolderIds->__union_NonEmptyArrayOfBaseFolderIdsType->__union_NonEmptyArrayOfBaseFolderIdsType = 2; //this indicates, which class/structure are taken from union
gftRoot->FolderIds->__union_NonEmptyArrayOfBaseFolderIdsType->union_NonEmptyArrayOfBaseFolderIdsType.DistinguishedFolderId = dfit; //push dfit to element
gftRoot->FolderShape = frst;
__ns1__GetFolderResponse response;
int ret = proxy->GetFolder(gftRoot, response);