pfSense

Currently, pfSense's help says that the username field should hold the "Dynamic DNS ID" (the same as the hostname), while the documentation suggests this should ("usually") be the account user/email, and in practice the value does not seem to be validated by the servlet at all.

According to some excerpts from https://dnsmadeeasy.com/technology/dynamic-dns/

username – The username you use to log into DNS Made Easy.
...
All arguments are required to make a successful call. From the DNS – Managed DNS menu, you can select a record and edit it to turn on Dynamic DNS and an option to set a password per record so you do not have to use your main DMS Made Easy password / username.
...
username (which is usually the users email account they use to log into DNS Made Easy)
...
https://cp.dnsmadeeasy.com/servlet/updateip?username=test@example.com&password=mypass&id=1007&ip=12.13.14.15

In practice, successful calls can be made with arbitrary values for the username field, or with the username field omitted entirely, e.g.

https://cp.dnsmadeeasy.com/servlet/updateip?password=mypass&id=1007&ip=12.13.14.15

Perhaps a reasonable reconciliation between the documentation and real life would be something like this:
- change the description of the username field to something along the lines of "usually the username or email used to log into DNS Made Easy"
- potentially, allow users to leave the field blank