WorkItem -> Links Tab -> Add button extension

I do not believe there is any extension point for this.  You will have to open an enhancement request.

I had to modify the rtc client source code to change the behavior of which link types where displayed.

what does 'add a menu' mean?  add a new linktype?  you can add linktypes already.

By 'Add menu' I mean to add a custom menu action to the list of menu actions already shown when you click on the Add button in the Links Tab of WI. The existing actions are Add Related,Make Duplicate Off etc. I am aware that I need to implement a plugin to customize this Add button. However I am unable to find any class of documentation which contains the details related to my requirement and also I am relatively new to RTC.