How can I set a webform's field value by query parameter?
This can be done in the webform backend settings: Edit your webform and go to Settings >> Form; then go down to Form Behaviours and select Allow all elements to be populated using query string parameters.

If you go back to the webform element, select the particular field, and enter [current-page:query:full_name] as default value (where full_name is the field name), if the query string contains full_name=John+Smith, the full_name field will be populated with Josh Smith.