js.js

const VM = DefineMap.extend({
  recipientPromise: {
    type: "any",
    value: function () {
      return Mail.get({pruefbericht_id: 1, foo: "bar"});
    }
  },
  recipient: {
    value: {},
    get: function (lastSet, resolve) {
      this.recipientPromise.then(resolve);
    }
  }
});

stache.stache

{{#if recipientPromise.isPending}}
    Loading...
{{/if}}
{{#if recipientPromise.isResolved}}
<div class="card-block">
    <div class="form-group row">
        <label class="col-sm-2 col-form-label">Empfänger</label>
        <div class="col-sm-10">
            <input type="text" class="form-control" {($value)}="recipient.mail" placeholder="E-Mail">
        </div>
    </div>
</div>
{{/if}}