Using WLPro 6.9, Windows 10.

In order to Short stocks or ETFs, you have to 'borrow' them from your broker. There is a cost involved for this called as the Borrow Rate (expressed as an annualized %).
The Backtest Settings have options for interest rate (for idle cash) as well as for Margin Loan. However, those are entirely different from the Borrow Rate.
I would like to see a separate box for Borrow Rate as an option (expressed as an annualized %) in a future WLPro upgrade in order to get more realistic results for Shorting.

I realize the Borrow Rate differs very widely from one stock to another, and can also vary for the same stock from time to time, yet its still better to have that option (where the user can input a reasonable value) than to not have one at all.

Not only it varies widely. As you know, some shares may not be available for shorting from time to time and from broker to broker. Now how do you handle this? So it's all a guesstimate.

- Margin rates also differ from time to time and from broker to broker, yet there's an option for that in Backtest Settings.
- And for sure sometimes shares are not available for Shorting - but WLPro still runs backtests assuming they were.
- A guesstimate is still better than assuming a ZERO cost.

Bottom line: It's CRUCIAL to have this option if one is to get meaningful results for Shorting!

You could try to emulate it with Slippage if you wish.

For a little extra work (the math already exists in the code from the Int. Rate & Margin Loan options) you have the opportunity to make WLPro a better, more complete product.

Calculating the Cost of Borrowing Stock at Interactive Brokers

Although examples on the page are dated 2017, it provides some insight on ETB vs HTB stocks. AAPL, being a prominent ETB, had a fee rate of 0.25% while SNAP's stock was HTB and the fee rate was a whopping 50% APR.

Still sure that a guesstimate between 0.25% and 50% is better than nothing?

It is up to the user to input a reasonable average value or run the backtest using multiple settings.
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Be positive.

It's a moot point. Thank you for your suggestion though.