How to set default values in ng-select in angular6?

if you are using both bindlabel and bindvalue so fist find index of selected value t e

var index= this.sensorTypes.findIndex(x => x.ID ==something); 
//this will set value
this.selectedAttributes= this.sensorTypes[index].ID;

You should use the [items] input binding instead of [options]

<ng-select 
  [items]="sensorTypes"
  bindLabel="label"                 
  [multiple]="true"
  placeholder="Select"
  [(ngModel)]="selectedAttributes">
</ng-select>

And on your component's module.ts, import the NgSelectModule. And if you haven't import your FormsModule, you should do so, as it needs to be imported for 2 way binding with ngModel to work.

import { NgSelectModule } from '@ng-select/ng-select';
import { FormsModule } from '@angular/forms';
.
.
@NgModule({
  imports: [
    FormsModule,
    NgSelectModule,
. 
.
.

values are not setting by default in multi-select

for this assign this.sensorTypes[0] to ngModel of your ng-select in ngOnInit()

    ngOnInit() {
      this.selectedAttributes = this.sensorTypes[0]
    }

this will get the first attribute as the default one.