Hi @jbhatt,
First of all to add custom tab to Change Request, you can add below snippets to respective files.
1. Below should go to "<WT_HOME>\codebase\config\actions\custom-actionModels.xml" file
<!-- Tabs for Change Request Info Page -->
<model name="changeRequestInfoPageTabSet" >
<submodel name="changeRequestDetailsTab"/>
<submodel name="changeRequestProcessTab"/>
<submodel name="changeObjectsHistoryTab"/>
<action name="sysnavTab" type="sysnav"/>
<action name="customTab" type="community"/>
</model>
2. Below should be in <WT_HOME>\codebase\config\actions\custom-actions.xml" file
<objecttype name="community" class="wt.change2.WTChangeRequest2" resourceBundle="" >
<action name="customTab">
<description>Custom Tab</description>
<title>Custom Tab</title>
<label>Custom Tab</label>
<command class="" method="" windowType="popup" url="netmarkets/jsp/custom/community/customTab.jsp" />
</action>
</objecttype>
Note: Please pay close attention to highlighted text above. As they have certain meanings while rendering custom tab.
3. Create a customTab.jsp under "WT_HOME/codebase/netmarkets/jsp/custom/community" folder
Note: If the folder hierarchy is not present, you may need to create it.
4. Add below to custom.jsp file.
<iframe src="https://www.ptc.com" height="600" width="1000"></iframe>
5. Stop Windchill Method Server
6. Clear Tomcat, Info*Engine cache
7. Restart Windchill Method Server
8. If required, you may also have to clear browser cache, to flush out cached pages.
Access Change Request page and you will see new tab added to CR displaying non Windchill page (third party).

I hope it will solve your purpose.
Regards,
Shirish