DKIM record is not accepting by Bind

When we setup a server by default dkim entry comes with the selector- "default". But when I want to  use a separate mail server (in my case I'm using mail-in-a-box server) then I need to put another txt (dkim) record with  "mail._domainkey" in cwp. but when I am doing the entry it says
"Error
There is some error with the configuration, please check and update"
if I add directly from the file (/var/named/example.com.db) then bind server stops working.
says
" dns_rdata_fromtext: /var/named/example.com.db:26: syntax error "

Already tried to add another dkim key with the selector"mail"
but nothing is working.

After some research:

Can not enter any dkim record which is more than 255 characters.

Any solution?

DKIM record (2048 bit) which is more than 255 characters.

contact cwp support to check it

Did you get any clarification in this issue? I'm having EXACTLY same problem. Using external mail server Mail-in-a-box, and need to add mail._domainkey with sha256 key. Getting an error when trying to add a TXT record.