I'm seeing other help articles, but it appears the plug-ins to make this possible require an additional cost. Are there any free options to do this?
JSU's 'Get started' documentation will guide you to the right direction:
Go through it by clicking 'Next'.
You will find the configuration of the Validators just next to the post functions.
No, without plugins you can only make a custom field mandatory in the field configuration scheme for a project, which applies to all screens.
If you just want the validation in a workflow transition, then only option is to go for add-ons like - Jira Suite Utilities.
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