//eventPage.js
var contextMenuItem = {
    "id": "grabText",
    "title": "Save text",
    "contexts": ["selection"]
};

chrome.contextMenus.create(contextMenuItem);

chrome.contextMenus.onClicked.addListener(function(clickData){
    if (clickData.menuItemId == "grabText" && clickData.selectionText){

        chrome.storage.local.set({snippet: clickData.selectionText}, function(){});
        chrome.windows.create({type: 'popup', focused: true, url: '/submitSnippet.html'}, function(){});

    }
})

//submitSnippet.js
  ...
$('#SnippetEntry').val(result.snippet);
  ...