I am trying to make a spreadsheet capable of opening an HTML file as a sidebar, but the HTML file is located on a master spreadsheet. Here is the code that I have already for opening the sidebar using an HTML that is in the spreadsheet.
function sidebar1()
{
var b1 = SpreadsheetApp.getActiveSheet().getRange('C13').getValue();
html = HtmlService.createHtmlOutputFromFile(b1).setTitle(b1);
ui.showSidebar(html);
}
It's only possible if you are the current user of that spreadsheet and that spreadsheet is the container of the script because SpreadsheetApp.getUi() as described in the documentation Returns an instance of the spreadsheet's user-interface environment that allows the script to add features like menus, dialogs, and sidebars. if a spreadsheet is opened by id or url then there is no user interface
I just thought that if you have two spreadsheets already open it might be possible to call a server side function on one to run a server side function on the other using Apps Script API which might be able to launch a Sidebar on the second sheet. So maybe I'm wrong. Give it a try.
One solution would be to open the HTML file into a temporary workbook, and copy the sheet from there into the workbook containing all of them.