I’m new to Shopify app developing

I create new app

npm init @shopify/app@latest

install it to shopify store, and now try send token to backend

i try this code :

import React, { useEffect, useState } from 'react';
import { getSessionToken } from '@shopify/app-bridge-utils';
import createApp from '@shopify/app-bridge';
import { Box, Card, Layout, Page, Text, BlockStack } from '@shopify/polaris';

export default function AdditionalPage() {
  const [iframeSrc, setIframeSrc] = useState('');

  useEffect(() => {
    // Parse the query parameters
    const params = new URLSearchParams(window.location.search);
    const apiKey = params.get('apiKey');
    const shopOrigin = params.get('shop');

    if (!apiKey || !shopOrigin) {
      console.error('Shopify app configuration is missing!');
      return;
    }

    // Initialize the Shopify app bridge
    const app = createApp({
      apiKey: apiKey,
      shopOrigin: shopOrigin,
      forceRedirect: true,
    });

    // Get the session token
    getSessionToken(app).then((token) => {
      const urlWithToken = `https://myurl?session_token=${encodeURIComponent(token)}`;
      setIframeSrc(urlWithToken);
    }).catch(error => {
      console.error('Failed to get session token:', error);
    });
  }, []);

  return (
    <Page>
      <ui-title-bar title="Panel" />
      <Layout>
        <Layout.Section>
          <Card>
            <BlockStack gap="300">
              <Text as="p" variant="bodyMd">
                This is an example of how to include an iframe within your Shopify app.
              </Text>
              <iframe
                src={iframeSrc}
                style={{ width: '100%', height: '500px', border: 'none' }}
                title="Dashboard"
                loading="lazy"
              />
            </BlockStack>
          </Card>
        </Layout.Section>
      </Layout>
    </Page>
  );
}

function Code({ children }) {
  return (
    <Box
      as="span"
      padding="025"
      paddingInlineStart="100"
      paddingInlineEnd="100"
      background="bg-surface-active"
      borderWidth="025"
      borderColor="border"
      borderRadius="100"
    >
      <code>{children}</code>
    </Box>
  );
}

but there is no token, how i should get it ? (or autorise backend – need token and shop domain)