Arrow Mufflers for the 2019+ KTM 690 Enduro/SMC-R require a 'link' pipe to adapt to either the stock or Arrow header. The reason for this is because they use the same front caps on many different models of their mufflers and 'adapt' them with a link pipe to the stock or Arrow header. If you do not already have an Arrow muffler, see one of the links below and add the Link Pipe as an option on that page.
See these links for available Arrow 690 Enduro/SMC-R Mufflers:
Warranty fulfilled by Arrow Exhaust.