Perform the following steps to make the Reason for Freezing field required during the Frozen transition:

1. Select and edit the Simple Workflow.
2. Select Diagram mode.
3. Click on the Frozen global workflow transition.
4. Click on the Validators link from the panel on the right-hand side.
5. Click on Add validator, select Fields Required (JSU) from the list, and click on Add.

6. Select the Reason for Freezing field from Available fields and click on Add >>. This will add the selected field to the Required fields list, as seen in the next screenshot:

7. Enter an optional error message if you would like to display a customized message instead of the default error message.
8. Click on the Add button to complete the validator setup.
9. Click on Publish Draft to apply the change.

After you have added the validator, if you try to execute the Frozen transition without specifying a value for the Reason for Freezing field, Jira will prompt you with an error message, as shown in the following screenshot: