Man pages matching name curlopt_proxyuserpwd in section 3:

CURLOPT_PROXYUSERPWD.3.en	debian 10 / libcurl4-doc
CURLOPT_PROXYUSERPWD.3.en	debian 11 / libcurl4-doc
CURLOPT_PROXYUSERPWD.3.en	debian 12 / libcurl4-doc