How to force form validation before modal will close

I have an Angular modal form that I'm styling with Bootstrap. I I want to validate when the user clicks "I Agree." (I don't want to submit the form because it is part of a larger form that will be submitted later.) How can I force this validation when the button is clicked? Also, should the form name be the same as the larger form or have its own name?

Modal:

<form name="agreementForm" novalidate>
    <div class="form-group" ng-class="{
    'has-error':agreementForm.signature.$invalid && !agreementForm.signature.$pristine
    }">

      <label class="control-label" for="signature">Signature</label>
      <input type="text" id="signature" name="signature" ng-model="agreementForm.contact.signature"
             placeholder="Signature (e.g., /John Doe/)" class="form-control" ng-minlength=1 ng-model-options="{ updateOn: 'blur' }" required />

      <span ng-show="agreementForm.signature.$error.required && !agreementForm.signature.$pristine" class="help-block">
               Please type your signature to agree</span>

    </div>

    <div class="modal-footer">
      <button ng-click="$dismiss()" class="btn btn-warning">Cancel</button>
      <button ng-click="$close()" class="btn btn-primary">I agree</button>
  </div>
  </form>

Would ng-submit solve your issue? For instance,

<form ng-submit="$close()">
    <input type="text" ng-model="signature" ng-required="true"/>
    <button type="submit">Submit</button>
</form>

It prevents the request being sent to the server and should still perform the validation on the form.