How can I retrieve a referral code from a Telegram bot's start parameter using a JavaScript landing page and send it to a Laravel backend?

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I am developing a Telegram mini-app that allows users to refer others and earn rewards. Although I have successfully implemented authentication and several functionalities, I am struggling to extract the referral code from the referral link, such as ‘Telegram: Contact @my_bot’. I’ve attempted various methods without success. Could someone please assist me in resolving this? Thank you! Below is my JavaScript code:

const telegramApi = await initializeTelegramSdk();
if (telegramApi) {
    const { id, firstName, userHandle, startParameter } = telegramApi.initDataUnsafe;
    console.log('User Information from Telegram:', { id, firstName, userHandle, startParameter });

    const csrfToken = document.querySelector('meta[name="csrf-token"]')?.getAttribute('content');

    const apiHeaders = {
        'Content-Type': 'application/json',
    };

    if (csrfToken) {
        apiHeaders['X-CSRF-TOKEN'] = csrfToken;
    }
    
    const apiRequestBody = {
        ...telegramApi.initDataUnsafe,
        referralCode: startParameter // Adding the referral code to the request body
    };

    const apiResponse = await fetch("{{ route('tg.authenticate') }}", {
        method: 'POST',
        headers: apiHeaders,
        body: JSON.stringify(apiRequestBody) // Sending the referral code in the payload
    });

    const apiData = await apiResponse.json();
    if (apiData.message === 'Logged in successfully' || apiData.message === 'User created and logged in') {
        console.log(`Hello ${firstName || userHandle}!`);
        setTimeout(() => {
            window.location.href = '{{ route('tg.dashboard') }}'; // Redirecting after 1.5 seconds
        }, 1500);
    } else {
        console.error('Login unsuccessful:', apiData.message);
    }
}

In my Laravel controller, I utilized Log::info('Request Input:', $request->all()); to verify the input, but the JavaScript is not transmitting the referral code.