os-ddclient - does not accept a ClouDNS apikey

[sja1440]

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Describe the bug
Since version 1.13.0, the 'Dynamic DNS' plugin (os-ddclient) has declared support for ClouDNS.

Updating the ClouDNS api does not use Username and Password. Instead it uses an API key as follows:
https://ipv4.cloudns.net/api/dynamicURL/?q=MYAPIKEY

1. It is not clear from the GUI where to insert the ClouDNS API key

2. Making many attempts at inserting the APIkey and url (parts of the url) into various combinations of the GUI fields:
   Username
   Password
   resourceid (using advanced mode)
   Hostname= mydomain.eu
   always fails with the same type of log entry (see below)

To Reproduce
Using the plugin's GUI I have tried

Expected behavior
The ClouDNS account should be accepted by the plugin and update successfully.

Screenshots
not applicable

Relevant log files
Here is a typical plugin log entry whenever I attempt to add a ClouDNS account (nb I have replaced my domain with the string MYDOMAIN):

2023-05-05T15:07:22	Notice	ddclient[8125]	9764 - [meta sequenceId="4"] WARNING: skipping host: MYDOMAIN.eu: 'dynurl=' is an invalid string.

Additional context
Add any other context about the problem here.

Environment
OPNsense 23.1.7_3-amd64

[AdSchellevis]

It was added in #3190 , I don't see any special treatment in the configuration file, maybe best check what ddclient expects first.

[sja1440]

I have monitored the contents of /usr/local/etc/ddclient.conf whilst attempting to register a ClouDNS account using the plugin's GUI.

No matter how I complete the plugin's GUI I see no "dynurl" field within the ddclient.conf.

An example of how it should be filled in is given in the /usr/local/etc/ddclient.conf.sample. This is what it says:

##
## ClouDNS (https://www.cloudns.net)
##
# protocol=cloudns, \
# dynurl=https://ipv4.cloudns.net/api/dynamicURL/?q=Njc1OTE2OjY3Njk0NDM6YTk2, \
# myhost.example.com

I therefore suspect that there is a bug in the GUI rather than the external ddclient script.

[AdSchellevis]

Let's tag this help wanted, shouldn't be very difficult to add.