How to pass paramter from view to vue root to component?

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Javascript

vue.js

i am trying to pass a string from Vue root to component, i am rather new to vue and not sure how to correctly pass data.

the parameter i am trying to pass is: userId

View: Index.cshtml ( this is where i get my parameter )

@using Microsoft.AspNetCore.Identity
@inject SignInManager<User> SignInManager
@using LaBouteilleDamour.Domain.Models;
@inject UserManager<User> UserManager

@if (SignInManager.IsSignedIn(User))
{
    User user = await UserManager.GetUserAsync(User);
    var userid = UserManager.GetUserId(User);

    <div id="cartApp" userId:"userid"></div>
    <script src="./js/Cart.bundle.js" asp-append-version="true"></script>
}

Vue root: Cart.boot.ts

import Vue from "vue";
import Cart from "./Components/Cart.vue";

new Vue({
    el: "#cartApp",
    template: '<Cart :userId="userId" />',
    props: {
    *userId: String,
    },
    components: {
        Cart
    }
});

Vue Component: Cart.vue ( where i need the parameter to go )

<template>
 /*HTML*/
</template>

<script lang="ts">
    import ShoppingCartItem from "../Components/ShoppingCartItem.vue";
    import ShoppingCartService, { ICartItem } from "./AP
/ShoppingCartService";
    import Vue from "vue";


    interface IShoppingCartpageData {
        items: ICartItem[],

    }

    export default Vue.extend({
        data(): IShoppingCartpageData {
            return {
                items: [],
            }
        },
        props: {
            userId: {
                type: String,
                required:true,
            }
        },
        ...
    })
</script>

What I have tried:

Variety of props and ways to access them, but i have no idea..