startYearMonth: [','],

this.$set(
  this.formModel,
  'startYearMonth',
  `${this.formModel.startYear}-${this.formModel.startMonth}`
);

 this.$set(
  this.formModel,
  'startYearMonth', [
  this.drawerModel.projectStartTime,
  this.drawerModel.projectStartTime,
]);