How to set the $APP_ROOT value using grunt?

I followed the of camunda-webapps and build the code successfully using “grunt” for camunda 7.16.

and I tried to host these web ui script using a web server, for example,
python -m http.server 8088

In this case, I opened the welcome page and type

however, it was halted.

It seems that it had a try to connect the camunda backend restful API.
In this case, how do i sperately specify the backend $APP_ROOT value when building UI using grunt
to customize the backend url and port ? Or could you share us the other solutions? Thank you!