How can a Jenkins user authentication details be "passed" to a script which uses Jenkins API to create jobs?

Try this way: (for example delete the job)

curl --silent --show-error http://<username>:<api-token>@<jenkins-server>/job/<job-name>/doDelete

The api-token can be obtained from http://<jenkins-server>/user/<username>/configure.

api curl jenkins token

This worked for me:

curl -u $username:$api_token -FSubmit=Build 'http://<jenkins-server>/job/<job-name>/buildWithParameters?environment='

API token can be obtained from Jenkins user configuration.

api curl jenkins token

With Jenkins CLI you do not have to reload everything - you just can load the job (update-job command). You can't use tokens with CLI, AFAIK - you have to use password or password file.
Token name for user can be obtained via http://<jenkins-server>/user/<username>/configure - push on 'Show API token' button.
Here's a link on how to use API tokens (it uses wget, but curl is very similar).

CodeHunter