diff --git a/site/en/guides/border-router/prepare.md b/site/en/guides/border-router/prepare.md
index b5ed0a93..3254b3f1 100644
--- a/site/en/guides/border-router/prepare.md
+++ b/site/en/guides/border-router/prepare.md
@@ -12,6 +12,8 @@ as a Full Thread Device (FTD) in the design you chose.
 * A Raspberry Pi for the Thread border router.
 * 2 Nordic Semiconductor nRF52840 USB Dongles (one for the Co-Processor and one for the Thread end device).
 
+When building firmware for the nRF52840 USB dongles, you must use the flag `-DOT_BOOTLOADER=USB`. This flag is required for both RCP and NCP designs to configure the correct memory map for the pre-installed Bootloader and to enable the USB DFU trigger. If the flag is not present, the compiled firmware cannot be loaded onto the dongle.
+
 ## Step 2: Build and flash Co-Processor firmware
 
 Follow the instructions based on the design you chose.