the function is called but the value send to the server is the initial input value.
Server filters work in a different manner. They just reload the data based on the Datatable state (which is returned by the getState method), and it always contains actual filter values.
As a simplest solution, you can block the onBeforeFilter event and load the data by API: https://snippet.webix.com/qd66rmjf
Or, a bit more advanced technique, define a loading proxy to modify request parameters just before it is issued: https://snippet.webix.com/yzyn5o9d
It looks like you're new here. If you want to get involved, click one of these buttons!