As the topic says if I have to define variable in index.html file how do I access if from event sheet?
Develop games in your browser. Powerful, performant & highly capable.
If you mean you have a JS variable you need to get from the parent page, since HTML doesn't have variables, you would right click in an event and choose add script, then
runtime.globalVars.myGlobalVar = variableFromWebsite
Assuming you have a global variable in your project called myGlobalVar, and your JavaScript has a defined variable called variableFromWebsite. This would set myGlobalVar to variableFromWebsite, which you can then use in your project as normal.
See editor.construct.net for more examples of how to integrate JS in the event sheet, via local variables. There is also a construct.net/en/forum/construct-3/scripting-51 forum you can ask questions on with people much much more familiar with JS than I am.