This solution worked for me, with some tweaking. I’ve posted my working code (with different values) below, hope this messy command helps somebody. Note that the space in hello world had to be filled in, and the double quotes are escaped.
npx aws-api-gateway-cli-test --username admin@example.com --password Passw0rd! --user-pool-id us-east-2_12345123 --app-client-id 6912345123451234512345 --cognito-region us-east-2 --identity-pool-id us-east-2:12341234-d614-4a0b-9318-1234512345 --invoke-url https://1234512345.execute-api.us-east-2.amazonaws.com/prod --api-gateway-region us-east-2 --path-template /notes --method POST --body {\"content\":\"hello_world\",\"attachment\":\"hello.jpg\"}