How to enable/disable field in Designer 4.02?

Hi! I try to enable/disable TrueFalseCheckbox field. I need it to be disabled on onload event and then I want to enable it on the custom event. So, I have tried two approaches. The first is I set an Editable property of the field to false and then I tried to enable field by field.enable(). It doesn't work. The field is still disables. The second approach is I set an Editable property of the field to true. Then on onload event I set it to disable: field.disable(). But field is still enabled.
It seems like the field listen to Editable property only, but not listen to field.enable(), field.disable().
Any suggestions?
Thanks!!!