Field to be made read only on start of a workflow

Hi Experts

I have a requirement wherein I need to make a field read only once the button to start the workflow is triggered.

I have gone through some blogs here for making a field read only by enhancing the GET_DATA method of the feeder class,

but this is not working in my case . I tried setting the ct_usage field read_only for my field ,also ev_field_usage_changed = X.

The required field is been first added to the BOPF structure then added to the FPM screen configuration. All other standard fields in the structure which are also there in screen gets into display mode once the workflow is triggered only the custom field remains in edit mode.

BOPF object : EHHSS_RAS_ACTION .

Any suggestion would be appreciated .

Thanks