Webform

You want:

?edit[submitted][webform_field_name]=Happy text value here

where "webform_field_name" is the name of the field in your form. Have a look at the name and id attributes for the input element for which you're trying to supply a value, and mash them up a bit using this pattern.

You can also just use webform's built-in capability to prepopulate a form field from the url by setting %get[key] as the standard-value of a field and then calling the webform with /webform-url?key=value. No need for the prepopulate module then.

I don't know what version this was originally opened for (the version # was blank) but since Prepopulate works with Webform, I thought we should close it. Leaving the issue open makes it appear to someone browsing the issue queue as if there's no webform support.