The DKIM needs to be added to your mail server (your hosting provider or mail provider can help with this).
To add a TXT DKIM record:
1. Log in to the Cloudflare dashboard.
2. Ensure the website you want to update is selected.
3. Select the DNS app.
4. Under DNS Records, complete the fields for your TXT record, including your DKIM values record (see image below).
5. Click Add Record.
DKIM records can often exceed the 255-character limit for TXT records. Most DNS providers, including Cloudflare, will automatically split these into multiple records at the same domain name, producing a record that looks like this in dig/nslookup:
default._domainkey.example.com. 299 IN TXT "v=DKIM1; k=rsa; p=<encoded public key>" "<rest of public key>;"
You should remove the quotation marks and the spaces between them when adding DKIM records to your zone.
You do not need to escape semicolons for your DKIM records on Cloudflare.
If you still encounter issues copy-pasting the DKIM record values, you could also try importing a zone file and then remove
To test DKIM records, there are several validation tools available online, but keep in mind these tools are often incorrect. One tool seems to be better at testing both in terms of the validity of the test results and the additional information associated with it for troubleshooting (http://dkimcore.org/tools/).
However, the recommended way to test is to look up the records using a dig command: https://scottlinux.com/2012/10/27/how-to-fetch-dkim-records-from-dns/.