You can easily auto-fill the fields on a form by adding a querystring to the end of the URL of your landing page. Here's an example:
Form embedded on a website:
Boast landing page:
You can find the correct parameter (email, mobile_phone, rating) to use by editing a form field, clicking on Advanced, and noting the System Name.
Primary fields will always have the same parameter names:
For advanced fields, you will need to check in the System Name field to see what the parameter name is.
Parameter names for a field are generated the first time you save the form with the field added.