To be considered valid YAML, you must wrap the entire command in single quotes. If the command already uses single quotes, you should change them to double quotes (") if possible:

job:
  script:
    - 'curl --request POST --header "Content-Type: application/json" "https://gitlab/api/v4/projects"'`

This Stackoverflow got bit more info
https://stackoverflow.com/questions/3790454/how-do-i-break-a-string-in-yaml-over-multiple-lines/21699210#21699210

Starting in GitLab 16.7 and GitLab Runner 16.7, you can now enable a feature flag titled FF_SCRIPT_SECTIONS, which will add a collapsible output section to the CI job log for multi-line command script blocks. This feature flag changes the log output for CI jobs that execute within the Bash shell.

A pipeline with a multi-line command in the script block for the build-job

variables:
FF_PRINT_POD_EVENTS: "true"
FF_USE_POWERSHELL_PATH_RESOLVER: "true"
FF_SCRIPT_SECTIONS: "true"

this will be helpful when you have multi line script and want to see the complete output.