There's only limited options you can do with JIRA custom fields based on its workflow. Some of it are on the link below
https://confluence.atlassian.com/jira/workflow-properties-189949.html
There are add-ons that can do what you want, but I'm not sure if they are available in the cloud.
https://marketplace.atlassian.com/plugins/com.onresolve.jira.plugin.Behaviours/server/overview
Behaviours was merged into Script Runner a while ago. It's not available for Cloud. (The other one that is worth a look is the Field Security add-on. I'm not sure that will do it, but it might. But, again, not for Cloud)
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