I want my custom field to be read-only

[koutarou ishibashi]

Is it possible to enter a value in a custom field and then prevent anyone from editing it?

[Robert Wen_Cprime_]

So what you have to do is set up three screens.  You define your screens in the screen scheme for the separate actions your issue can go through (Creste, edit, and view).  You then put your custom field in the create and view screens only.

This may help: https://support.atlassian.com/jira-cloud-administration/docs/manage-screen-schemes/

[koutarou ishibashi]

Thank you for viewing the view screen and can no longer be edited.

Is it possible to make it impossible to edit the screen displayed during the transition only by displaying?

[Robert Wen_Cprime_]

You can use workflow properties to set up restrictions on the issue for a status.  What you want is to set jira.issue.editable to false on specific statuses.

This should help: https://support.atlassian.com/jira-cloud-administration/docs/use-workflow-properties/