The demo was first sent to Elvis Presley, who revealed this during the Million Dollar Quartet jam session in December 1956.
As he recalled, "Have you heard Pat Boone's new record, called 'Don't Forbid Me'?
Presley then took the acoustic guitar he was playing and ran down a couple of terrific versions by himself to demonstrate to the others how he might have done it.
According to eminent author Mark Lewisohn in "The Complete Beatles Chronicles" (p. 362), The Beatles performed Don't Forbid Me on stage from at least 1960 and through 1961, which would have been in Liverpool and Hamburg, with Paul McCartney on lead vocal, but no recording is known to survive.
However, their performance of it on stage in Hamburg is probably where big-band conductor, arranger, writer and producer Bert Kaempfert heard the song, as he had previously never covered a single rock song of any sort for his instrumental records.