This HN comment[1] enlightened me.

=> https://news.ycombinator.com/item?id=9447636 1: https://news.ycombinator.com/item?id=9447636

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To download a file:

curl -u david:$(op item get flounder --fields label=password) \
  -O sftp://flounder.online:2024/index.gmi

To upload a file:

curl -u david:$(op item get flounder --fields label=password) \
  sftp://flounder.online:2024 --upload-file index.gmi