https://www.pbainbridge.co.uk/2018/12/get-activecell.html

Get ActiveCell.gs

function getActiveCell() {
  
  // get the active spreadsheet and sheet
  var ss = SpreadsheetApp.getActiveSpreadsheet(); // ss contains entire spreadsheet
  var sheet = ss.getActiveSheet(); // sheet contains current active sheet ('Students')
  
  // from sheet, get active cell and store as variable
  var activeCell = sheet.getActiveCell();
  
  // from activeCell, get value and store as variable
  var cellValue = activeCell.getValue();
  
  // show value of cellValue
  Logger.log(cellValue);
  
  // from activeCell, also show row number
  var cellRow = activeCell.getRow();
  Logger.log(cellRow);
  
}