Paste the command output:
command | curl -sF "plain=<-" https://plain.im/
Create an alias:
alias plain='curl -sF "plain=<-" https://plain.im/'
to shorten paste command to just
command | plain
Paste and make link available as an X selection for pasting into X applications (requires xclip):
alias plain='curl -sF "plain=<-" https://plain.im/ | tee /dev/stderr | xclip'
Paste can be removed by making a DELETE request to the paste URL:
curl -X DELETE https://plain.im/key'