There’s no built-in auto increment possibility within the library.
As a simplified solution, you can use .getLastId() method for getting the id of the last row, but since it depends on current order it will fail on sorting.
So, you have to iterate through all the rows to find the biggest value. You can use Datastore .each method for these needs: http://webix.com/snippet/bceacb06
Rather than this, you can even use:
var recIdsArray = datatable.data.order;
var lastRec = Math.max(…recIdsArray);
This should give you the lastRec id. Looping thru all the recs just to find the max value takes lot of time and makes it tedious if you have a million of records in store.