RTC 6.2: How to execute server plugin from client side

You can't for all I know.

The idea is, instead of value coming from RTC should come from the web service in the work item attribute each time user open to tab.

1. It can only be done something with Scripting support to invoke the server side plugin that will set the attribute value.

This whole approach completely ignores how the RTC UI's work and what they support in terms of extending. It also ignores that RTC has also an Eclipse UI and how that works and what it allows.

The only built in mechanism I am aware of to get external values from a web Service is the HTTP filtered value set. The only way to set values dependent on some change event is calculated values or follow up actions. The only extension point I am aware for attribute values and editor presentations is custom editor presentations where you could create some custom attribute value picker for a cost.

You can do nothing based on changing a tab, because there is no extension point.

If you want a reasonable insight into what you can do to extend RTC see

RTC Process Customization – What you can and cannot do