Changing-symbol-for-the-secondary-chart

I think the most straightforward way is to create a Dictionary and hard-code the symbols in the script. If the list is long, then you could load the dictionary from a file.

Note that in the code, the symbols "on the left" are the "keys" to the Dictionary. They must be unique. On the right is the secondary symbol you assign to them - these can be duplicates.

CODE:
using WealthLab.Backtest;
using System;
using WealthLab.Core;
using WealthLab.Indicators;
using System.Drawing;
using System.Collections.Generic;

namespace WealthScript1 
{
    public class MyStrategy : UserStrategyBase
    {
        
        public override void Initialize(BarHistory bars)
        {
         _lookup = new Dictionary<string, string>();
         _lookup["AAPL"] = "#RUT%MA50";
         _lookup["TTD"] = "#RUT%MA50";
         _lookup["ZS"] = "QQQ";
         _lookup["IBM"] = "SPY";
         _lookup["SIRI"] = "QQQ";
         _lookup["CSCO"] = "SPY";

         _secondsym = _lookup[bars.Symbol];
         
         Color color50d = Color.FromArgb(255, 230, 70, 60);
         PlotTimeSeriesLine(SMA.Series(bars.Close, 50), "SMA50", "Price", color50d, 1);
         PlotTimeSeriesLine(SMA.Series(bars.Close, 200), "SMA200", "Price", Color.Black, 1);

         // Stocks in Russell 2000 above 200 MA
         BarHistory bars2 = GetHistory(bars, _secondsym);
         PlotBarHistoryStyle(bars2, "paneBottom", "line", color50d);
         DrawHeaderText("Percent of Russell-2000 Stocks above 50 day MA", color50d, 14, "paneBottom");
        }

        //execute the strategy rules here, this is executed once for each bar in the backtest history
        public override void Execute(BarHistory bars, int idx)
        {
            if (!HasOpenPosition(bars, PositionType.Long))
            {
                //code your buy conditions here
            }
            else
            {
                //code your sell conditions here
            }
        }

      //declare private variables below
      Dictionary<string, string> _lookup; 
      string _secondsym; 

    }
}

So we've got three ways and counting 😃

Thank you Eugene and Cone!!!
Both are good ideas!

On the UI popup, I have to think more on a way to streamline it with my workflow.

The symbol mapping will work if I have only one-to-many relations from primary symbol to secondary symbol.
I think this feature will make it work for all combinations (many-to-many) - https://www.wealth-lab.com/Discussion/Add-Symbol-Mappings-for-Historical-Providers-7000
I have added comment in that thread and voted it!