it does not seem to show when I send emails
Well, you don't have anything which would add it.
SPF and DMARC records are self-contained, they only indicate some policy and do not change the message itself. DKIM works differently, it requires that the "sending" mail server actually sign the message using the private key and attach the signature as a header. (It's similar to PGP signatures, but usually done by the server, not by the mail app.)
So you must put the private key on the mail server. If you've found out that 1&1 doesn't support custom DKIM keys, your only other option is to run your own SMTP server for Thunderbird; make it sign messages with the DKIM key; then (probably) relay them to 1&1's SMTP server for actual delivery to the recipient.
(The 'relay to 1&1' part isn't strictly required, but it'll let you avoid additional hassle of trying to earn spamfilters' trust.)
DMARC is not a direct alternative to DKIM. It's a policy record that makes the verification of existing DKIM and SPF data a bit stricter (e.g. requiring all domains to match), but does not actually provide the digital signatures themselves.
(Perhaps the provider is confusing DMARC with ARC? The latter is a derivative of DKIM with more features, but is currently very rare outside of Gmail.)
What email system do you use and did you configure it to add the DKIM header correctly to your outgoing mail? – StarCat – 2019-06-07T09:50:39.360
@StarCat I am quite new to this so forgive me if I go round in circles with my explanations :-). My emails go through 1&1 Ionos webmail and I use thunderbird as my email client – SupGen – 2019-06-07T19:30:52.503
@StarCat I have just found, after various phonecalls, that 1&1 Ionos do not support DKIM Records in their webmail... My record is shown when I do a test for the specific selector that I have set in the DNS but it does not seem to show when I send emails which has left me confused... They say they offer DMARC as ana lternative which I have also set up. Is DMARC a comparable alternative for verifying emails? – SupGen – 2019-06-07T19:36:31.573