Environment variable

PASSWORD="mypassword"

Hooks

import { PASSWORD } from '$env/static/private';
import { redirect } from '@sveltejs/kit';

export const handle = (async ({ event, resolve }) => {
    if (!event.url.pathname.startsWith('/password')) {
        const password = event.cookies.get('password');
        if (password !== PASSWORD) throw redirect(307, '/password');
    }

    const response = await resolve(event);
    return response;
})

Password page

<form method="POST">
	<input type="password" name="password">
	<button type="submit">Submit</button>
</form>

import { PASSWORD } from '$env/static/private';
import { redirect } from '@sveltejs/kit';

export const actions = {
    default: async ({cookies, request}) => {
        const data = await request.formData();
        const password = data.get('password')?.toString() || '';

        cookies.set('password', password, {
            path: '/',
        });

        if (password == PASSWORD ) throw redirect(307, '/');
    }
}