this seems to work

linux/fsfwconfig/ipc/MissionMessageTypes.cpp

@@ -0,0 +1,10 @@
+#include "MissionMessageTypes.h"
+
+#include <fsfw/ipc/CommandMessage.h>
+
+void messagetypes::clearMissionMessage(CommandMessage* message) {
+  switch (message->getMessageType()) {
+    default:
+      break;
+  }
+}

linux/fsfwconfig/ipc/MissionMessageTypes.h

@@ -0,0 +1,22 @@
+#ifndef FSFWCONFIG_IPC_MISSIONMESSAGETYPES_H_
+#define FSFWCONFIG_IPC_MISSIONMESSAGETYPES_H_
+
+#include <fsfw/ipc/FwMessageTypes.h>
+
+class CommandMessage;
+
+/**
+ * Custom command messages are specified here.
+ * Most messages needed to use FSFW are already located in
+ * <fsfw/ipc/FwMessageTypes.h>
+ * @param message Generic Command Message
+ */
+namespace messagetypes {
+enum MESSAGE_TYPE {
+  MISSION_MESSAGE_TYPE_START = FW_MESSAGES_COUNT,
+};
+
+void clearMissionMessage(CommandMessage* message);
+}  // namespace messagetypes
+
+#endif /* FSFWCONFIG_IPC_MISSIONMESSAGETYPES_H_ */