This is a bit trickier customization as it requires overriding the HtmlField's LinkManagerDialog. I have prepared a sample custom dialog attached to this reply, so you can use it on your end. Extract the files into the root of your project, build and then test it.
Basically we are inheriting from the LinkManagerDialog and using the following approach:
we are registering a custom template to define the additional control for the Subject field:
and a custom script in order to operate with the input of the field. We are getting the value of the Subject field and adding it to the href attribute of the anchor tag that gets generated.
In order for Sitefinity to use the custom dialog we are replacing the built in one from the Global.asax like so:
sender, EventArgs e)
Bootstrapper.Initialized += Bootstrapper_Initialized;
sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
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