SO example

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  model:{
    tips: [{name:"nameTips1"},{name:"nameTips2"},{name:"nameTips3"}],
    hiws:[{name:"nameHiws1"},{name:"nameHiws2"},{name:"nameHiws3"}]
  },
  actions:{
    tipChange(value){
      this.set('eventTip', value);
    },
    hiwChange(value){
      this.set('eventHiw', value);
    },
    save(){
      const event = this.get('store').createRecord('event', {
       hiws: this.get('eventHiw'),
       tips: this.get('eventTip')
  		});
      event.save();
    }
  }
});

application.template.hbs

<select onchange={{action 'tipChange' value="target.value"}}>
{{#each model.tips as |tip|}}
    <option class="tables__sidebar-option" value={{tip.name}}>{{tip.name}}</option>
 {{/each}}
</select>
<select onchange={{action 'hiwChange' value="target.value"}}>
{{#each model.hiws as |hiw|}}
    <option class="tables__sidebar-option" value={{hiw.name}}>{{hiw.name}}</option>
 {{/each}}
</select>
<br><br>
<b>eventTip:</b> {{eventTip}}
<br>
<b>eventHiw:</b> {{eventHiw}}
<br><br>
<button {{action 'save'}}>save</button>

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}