How set DKIM properly with ESP?

I follow this post : https://www.digitalocean.com/community/questions/dkim-on-dns-panel-digitalocean So I generate a private key and a public key. In digitalocean DNS I put a record TXT with mail._domainkey and the part between “v=DKIM1; k=rsa; p=AIAfA…”. To send mail I use Django and I use an external email service provider…so in Django backend code I log with e-mail, password of the email account and send a mail with the comand send_mail(). On the ESP side I insert the same record TXT. It seems not work and DKIM fail (maybe I miss something in the DNS configuration in both side?)…I didn’t understand if I need to create a simple local server mail (for example using postfix) to sign the mail and after send it to the ESP or I can bypass this part. For the moment I use the TXT record provide from the ESP (I use it in a TXT record on digitalocean) in this way my email will sign by the ESP not by me… for example yahoo receive an email from me (from my esp but wrote by me), he read the selector and my domain in the header and so yahoo mail make a dns query to check the sign and all works good.

If I want sign by myself the mail, what I need? I want anyway use an ESP to avoid complex configuration but also to have a mail interface.