Hi,
Thank you for your post!
I am sorry to tell you that a control that is ReadOnly or Disabled won't post its value being changed to the server.
The workaround is to add a hiddenField, also assign the value of the disabled textbox to the HiddenField via javascript when the postback is starting. Then get the hiddenField's value on server side.
If you have further questions,let me know.
Best Regards,