This task involves following redirects.
Let’s log in with creds test:test! and intercept this POST request:
Press Forward and see text in the URL that looks suspiciously like base64:
Continue pressing Forward and get next part:
cGljb0NURntwcm94aWVzX2Fs
bF90aGVfd2F5X2EwZmUwNzRmfQ==
picoCTF{proxies_all_the_way_a0fe074f}