for(var i=0; i < data_get.body["skill_levels"].length; i++){
  this.skill_levels.push({id: data_get.body["skill_levels"][i]["id"], tcode: data_get.body["skill_levels"][i]["name"]});
}

this.skillForm.controls["skill_level_id"].setValue(skill_level_id);



<ion-select value="" ok-text="Ok" cancel-text="Annulla" formControlName="skill_level_id"
                       style="white-space: pre-line !important" >

         <ion-select-option *ngFor="let skill_level of skill_levels" [value]="skill_level['id']">{{ skill_level['name'] }}</ion-select-option>
</ion-select>