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This will embed the form instance in your page inside an <iframe/>. 

How do I get the url to return the form instance embededr java script?

First get a hold of the corresponding FormTypeEntry and then get the url as shown in the snippet below:

FormTypeEntry formType = ...; // find the correct formtype entry

NameValueCollection nvc = new NameValueCollection();
nvc.Add("_data", "{TextArea3:TestTestTest}");

Uri formInstanceEmbedUri = formType.CreateFormInstanceEmbed(nvc, null);

String formInstanceUrl = formInstanceEmbedUri.AbsoluteUri;

Then using the formInstanceUrl above you can generate the following script tag in your html page:

  <script type="text/javascript"  
  src="{formInstanceUrl}"></script> 

This will embed the form instance in your page inside an <iframe/>. 

How do I get a form entry based on a known id?

This is often needed when you need to relate a  form with another concept your application (e.g. you application has the concept of a report that has an associated  form). In this case you will store the form id somewhere and when needed fetch the form entry to embed it in your page, delete it, etc.
Here is how you get the for a form entry (in fact any entry):

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