You cannot create readonly Custom Field in the way in which you are trying so that you can type in some text to remain read-only. It does not work in this way and we do not have this feature at present.
The readonly Custom Field is available for Technical to use as part of a set up option when setting up a row set up only inside custom fields (not any individual custom fields) as part of customisation work and work-flow processes.
To find out more please contact our Support Helpdesk.