function insertCopiedCells() {
 var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
 var copyRange = spreadsheet.getActiveRange();
 var copyRangeRows = copyRange.getNumRows();

 // This is where I want to select the range with a mouse selection

 var pasteRange = spreadsheet.getRange(rangeSelection);
 var pasteRangeRow = pasteRange.getRow();
 pasteRange.activate();
 spreadsheet.insertRowsBefore(pasteRangeRow, copyRangeRows);
 copyRange.copyTo(pasteRange);
}