You can't, natively. Jira doesn't do field level security.
The feature below was created to request this, and it was closed as 'Won't Fix'.
I would recommend to check the issue above for more details about the decision for Atlassian not implementing the feature, as well to check the workarounds provided in there.
You may also check the Field Security Plugin from Quisapps, as mentioned by Nic.
This other question also provides some additional alternatives to achieve what you want:
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