How to get SQL Server filetable path locator for subfolder by its literal path
I found a solution.
Here is example where filetable name is 'physical_files', target path is 'folder1', target file have stream_id = 'FF9CF452-B522-E411-A464-00259060BBB9'

declare @path varchar(MAX);
set @path = '\folder';

declare @parent hierarchyid;
set @parent = GetPathLocator(CONCAT(FileTableRootPath('dbo.physical_files'), @path));
select @parent

update physical_files set path_locator = @parent.GetDescendant(NULL, NULL)
where stream_id = 'FF9CF452-B522-E411-A464-00259060BBB9';