Tested on Snom 710 with firmware 8.7.5.35.

The Snom 710 has no offhook dialing / hotline functionality.

To make this work you need to use the off hook Action URL.

First go to: Advanced → QoS/Security
And remove the user and password entries from HTTP Server.
I have not been able to get this to work when a username/password is set on the HTTP Server.

To lessen the security impact of disabling HTTP Server username/password, go to:

Advanced → QoS/Security
Set a password for Administrator mode.
And set Administrator mode: off.
This requires you to authenticate within the web page for modifying Administrator settings.
Don't forget to switch Administrator mode to off each time after making other changes!

Also set Advanced → QoS/Security
Ignore security advices: on
To prevent the messages in the web interface and on the phone that the http password is not stored.

Now we can set the Action URL.
Go to Action URL Settings.
Set “ On offhook” to:

https://127.0.0.1/command.htm?number=12345678

Replace 12345678 with the number to call.

Downside:
The phone will ALWAYS call the configured number on off hook.
EVEN when answering an incoming call.

Sources:
Snom Wiki - FAQ/Do snom phones provide the "Hotline Functionality"
Snom Forum - On offhook: Dial to 911

The following had to be set when using HTTP Server authentication, but does not seem to be required without HTTP Server authentication:
Also set Advanced → QoS/Security
Restrict URI queries: off
To allow calling command.htm with a query.