unable to attach vscode path variables (${file}), etc.. to devtools,

Environment (please complete the following information):

• OS (e.g., Windows 10): ubuntu 20.04
• Extension version (e.g., 1.2.3): 1.4.6

Describe the bug:

unable to get my index.html to launch by default when "launch instance" is pressed, head/headless. problem encountered using relative vscode path variables: "${workspaceFolder}", "${file}".
i tried in both default entrypoint and default URL fields, keeping the other at default value. i also tried setting both. i tried setting url to workspace and entrypoint to "index.html"
Repro steps:

fresh install of extension,
open index.html in your project folder,
head to extension in activity bar,
click "more settings"
enter any vscode file variable with/without quotation marks such as "${workspaceFolder}" into url,
inter index.html into entry point
click the "+" button with hover status of "open new tab",
find blank screen with this url:http://%24%7Bworkspacefolder%7D/
Expected behavior:
open index.html

i wanted to load the contents of index.url by using the file url. the file url seems corrupt
Additional context:

Start by reproducing the issue in the extension's More Settings view with ${workspaceFolder} or ${file} in the URL or entrypoint fields, using the reported Ubuntu 20.04 and extension version 1.4.6 environment if available. Trace how those settings are passed when opening a new tab; done means a project index.html opens instead of a URL containing the encoded variable.