Server side code has to return plain data with parent child relation like this
ID parent FirstName LastName
1 2 Nancy Davolio
2 Andrew Fuller
3 2 Janet Leverling
4 2 Margaret Peacock
5 2 Steven Buchanan
6 5 Michael Suyama
7 5 Robert King
8 2 Laura Callahan
9 5 Anne Dodsworth
Similar server side code is provided in this example
https://paramquery.com/pro/demos/treegrid-lazy