I don’t use windows-terminal much. But I do find that I want it to be my WSL Ubuntu-18 profile instead of PowerShell. To set that up you just need to:

Get Profile ID for Desired Profile

Open Windows Terminal -> Settings this opens the settings.json

Windows Terminal Settings

Below is an example:

{
    "$schema": "https://aka.ms/terminal-profiles-schema",
    "profiles" : 
    [
        {
            "guid" : "{61c54bbd-c2c6-5271-96e7-009a87ff44bf}",
            "icon" : "ms-appx:///ProfileIcons/{61c54bbd-c2c6-5271-96e7-009a87ff44bf}.png",
            "name" : "Windows PowerShell",
            "startingDirectory" : "%USERPROFILE%",
            "useAcrylic" : false
        },
        {
            "guid": "{c6eaf9f4-32a7-5fdc-b5cf-066e8a4b1e40}",
            "hidden": false,
            "name": "Ubuntu-18.04",
            "source": "Windows.Terminal.Wsl",
            "snapOnInput" : true,
            "startingDirectory": "/mnt/e"
        },
        {
            "guid": "{46ca431a-3a87-5fb3-83cd-11ececc031d2}",
            "hidden": false,
            "name": "kali-linux",
            "source": "Windows.Terminal.Wsl"
        }
    ],

Note the guid for the profile you want to use.

Update the setttings.json defaultprofile

Add json name of defaultProfile with the guid of your chosen profile like below:

{
    "$schema": "https://aka.ms/terminal-profiles-schema",
    "defaultProfile": "{c6eaf9f4-32a7-5fdc-b5cf-066e8a4b1e40}",
    "profiles" :

Save and close.