UPDATE 2:

I put both scripts to the same js file and attached them to the list webpart through Miscellaneous -> Jslink (didn't use powershell for this).

The first approach will work with old items which don't have the value for TrackingStatus field. The second approach changes the value of the TrackingStatus field after you edit any other field in the same row and it works with old and new rows (you could also use some other event. Here's the list of all events supported by JSGrid).

Also, please double check your column name and change the script accordingly, since you wrote "Tracking Status" (with space) in comments, and in the question it was TrackingStatus.

UPDATE: There is another approach, that involves attaching the event to the jsGrid control, which checks for the field value and updates it if it's empty.

SP.SOD.executeFunc('clienttemplates.js', 'SPClientTemplates.TemplateManager.RegisterTemplateOverrides', function() {
        var alreadyAttachedEvent = false;
        SPClientTemplates.TemplateManager.RegisterTemplateOverrides({
            OnPostRender: function(ctx) {               
                if (ctx.enteringGridMode || !ctx.inGridMode || alreadyAttachedEvent)
                  return;
                var jsGrid = null;
                for(var v in g_SPGridInitInfo)
                {
                    var jsGridContainer = $get("spgridcontainer_" + g_SPGridInitInfo[v].jsInitObj.qualifier);
                    var jsGrid = jsGridContainer.jsgrid;
                    break;
                }

                jsGrid.AttachEvent(SP.JsGrid.EventType.OnCellEditCompleted, function(args) {
                    var rec = jsGrid.GetRecord(args.recordKey);
                    if (!rec.properties["TrackingStatus"].localizedValue)
                    {
                        var update = SP.JsGrid.CreateUnvalidatedPropertyUpdate(args.recordKey,'TrackingStatus',"In Progress",false);
                        jsGrid.UpdateProperties([update], SP.JsGrid.UserAction.UserEdit);
                    }
                });
                alreadyAttachedEvent = true;
            }
        });
 });