How to grey-out a custom field and display an automatically created value instead ?

[YM]

A pre-filled automatically created  (dynamic) custom filed  as result of a  script added to Jira .

How to grey-out this field and prevent users from entering any value into this field

 and display the prefilled automatically generated value in this greyed-out field?

[Nic Brough -Adaptavist-]

If you are on Server, then I'd suggest the use of a "scripted field".  If you're on Cloud, you can still script fields, but you'll also need to ensure you remove them from the "edit" screen so that people cannot edit them.