Sadly, is seems you can't get on Dev or Beta channel since June 24th (if you weren't enrolled before), so you won't be able to get the Insider build with WSLg on your existing win10. Only if you install it from clean ISO and update. Source: https://answers.microsoft.com/en-us/insider/forum/all/how-in...

I tried it now and you could only choose Release Preview channel which still has only build 19044.