diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4c18bb63a..a2a1efe1d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -152,6 +152,8 @@ list(APPEND OT_PUBLIC_INCLUDES ${PROJECT_SOURCE_DIR}/include)
 if(OT_PLATFORM STREQUAL "posix")
     target_include_directories(ot-config INTERFACE ${PROJECT_SOURCE_DIR}/src/posix/platform)
     add_subdirectory("${PROJECT_SOURCE_DIR}/src/posix/platform")
+elseif(OT_PLATFORM STREQUAL "external")
+    # skip in this case
 elseif(OT_PLATFORM)
     target_include_directories(ot-config INTERFACE ${PROJECT_SOURCE_DIR}/examples/platforms/${OT_PLATFORM})
     add_subdirectory("${PROJECT_SOURCE_DIR}/examples/platforms/${OT_PLATFORM}")
diff --git a/etc/cmake/functions.cmake b/etc/cmake/functions.cmake
index ffafe48ad..4cb0bef8c 100755
--- a/etc/cmake/functions.cmake
+++ b/etc/cmake/functions.cmake
@@ -28,7 +28,7 @@
 
 # Get a list of the available platforms and output as a list to the 'arg_platforms' argument
 function(ot_get_platforms arg_platforms)
-    list(APPEND result "NO" "posix")
+    list(APPEND result "NO" "posix" "external")
     set(platforms_dir "${PROJECT_SOURCE_DIR}/examples/platforms")
     file(GLOB platforms RELATIVE "${platforms_dir}" "${platforms_dir}/*")
     foreach(platform IN LISTS platforms)