Web Designer & Designer Pro has a number of form widgets integrated within the program. Open the Design Gallery and expand the folder labelled Form Widgets (Download)
The form widgets contains a number of form providers. Double click or drag and drop the required widget into your page. This will cause a preview window to open directing you to the website of the widget provider.
Create your form as required and click on the Insert button to close the browser window and insert the widget within your page. Web Designer will automatically generate a preview image of the widget inserted providing a wysiwyg representation of the published widget within your document.
Please also see this guide on how to insert your form: http://www.xara.com/widgets/365/forms/
You can also insert your own form or a form from another provider into a placeholder.