...you can't. NVIDIA's Remix disables some functions of the overlay (for now). But since it partially exists you need to launch any other Steam game on the background. Try to find anything small and light not to interfere with quite heavy process of RTX.

For some reason Lab Rat achievement is unlocked by default so you won't be able to obtain it. Let's fix it. Go to your local files of Portal RTX (Steam\steamapps\common\PortalRTX\portal_rtx) and open the "gamestate.txt" file. You will find there list of achievements (only one in case you didn't launch the game).

There is an achievement with id 115. Set "value" to 0. Now the game won't have "Lab Rat" achievement unlocked. Just play the game normally and you will have it as soon as you grab the upgraded portal device.