Proxy to Another Service

My goal is to go to the Dashboard from the Main Site while showing the Main Site’s URL with /dashboard. For some reason, all I receive is a blank page and a console error that doesn’t make any sense.

Main Site: https://sad-mestorf-27b692.netlify.app/
Dashboard: https://reverent-shockley-07925f.netlify.app/

Goal: https://sad-mestorf-27b692.netlify.app/dashboard actually shows the https://reverent-shockley-07925f.netlify.app/ but retains the URL.

_redirects

/dashboard    https://reverent-shockley-07925f.netlify.app/   200
/*    /index.html   200

I also do have Pretty URLs turned on.

Can you try /dashboard https://reverent-shockley-07925f.netlify.app/ 200! please?