Submit the df_preview parameter as part of the API call that your API donation form will make.

This will still evaluate the success of the API call, but it will not create a transaction in Luminate Online.

Please note, the fact that the df_preview parameter is included in the API call is what is pertient,
the value submitted is not important.  This means that submitting df_preview=false will still be treated as
a request for the preview behavior.