How to add external domain DKIM records

I have an External SMTP server and want to add DKIM of that domain with different selector.

But after saving the TXT record the "+" sign has been removed and contain "enter" character. The value of DKIM is under 255 characters, so it no need to split.

For further explain I added the records below:

I add below TXT record

v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDKmAOLmsa8SeP86gTRF4G6+iK34e2bo7Qp37LhcRDOeFlxdhm5N49iIa6TeV5HCQI2vXxkzlXGxVV60UbpKHNaf9ArWLvg8Goy1CyHD22d9nWL+QBShmnzg9uWJZXexd1P1dy4kOJRDbI3SyFanjOGQF+HVdmBrJdIRsrSGh9BEQIDAQAB

after click on save, showing below record

v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDKmAOLmsa8SeP86gTRF4G6 iK34e2bo7Qp37LhcRDOeFlxdhm5N49iIa6TeV5HCQI2vXxkzlXGxVV60UbpKHNaf9ArWLvg8Goy1CyHD22d9nWL QBShmnzg9uWJZXexd1P1dy4kOJRDbI3SyFanjOGQF HVdmBrJdIRsrSGh9BEQIDAQAB

Ironically, the clue to the solution is in your question - a number of times.
Try a search for "Split DNS record" (another clue.)

  Thank you very much cynique for your clue!

I manually edit record in the DNS Functions => List DNS Zone => mydomain.com.db file and restart the BIND DNS Server from Admin Panel.

Now the DKIM is validate without spiting because as I described above it is under 255 characters.

My problem is solved but it is still a question that's why it were not properly save from User panel.

You're welcome. 
The DNS editor is full of bugs. It can't be particularly easy to code for, with variable record length and fields (ex. MX record).
I've previously mentioned bugs with the trailing point used to terminate/delimit a FQDN. Just yesterday, I needed to manually edit a domain for the records to be corrected.
The GUI is a useful tool but nowhere near the polished cPanel version, which appears to have much in the way of syntax checking.
Think of it as a quick means to add in records, then go and check the file for obvious mistakes (missing TTL, for example).

I am also Administrate different cPanel Servers but new to Centos Web Panel and I appreciate the efforts of senior persons on this forum like you who spare quality time to educate and solve the problem of others.

Thank you once again for further clarify about DNS zone!