What version are you using? It should be exposed in the last ones:
https://github.com/rust-nostr/nostr-sdk-js/blob/v0.42.1/src/client/mod.rs#L558
nostr:nprofile1qqsx3kq3vkgczq9hmfplc28h687py42yvms3zkyxh8nmkvn0vhkyyuspz9mhxue69uhkummnw3ezuamfdejj7qg4waehxw309aex2mrp0yhxgctdw4eju6t09uq32amnwvaz7tmwdaehgu3wdau8gu3wv3jhvtc8quffm I'm playing around with sendPrivateMsg in nostr-sdk-js thinking it behaves like the Python counterpart where I can give rumor_extra_tags but there isn't an equivalent option. Is that on purpose or maybe it's not built yet?
Discussion
That was exactly my issue. I didn't have the latest for whatever reason.