How do I set a default value for input field?

when try to add default values to my input and I select the image which need to upload into the database input values are hidden when submitting the from there is an some error display like this

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'student_id' cannot be null (SQL: insert into `payments` (`image_path`, `student_id`, `course_id`, `st_id`, `updated_at`, `created_at`) values (images/nSdUrRgdchUSISbrwW1ExkAON6RaAmNyK4tbVgHy.jpg, ?, 1, ?, 2021-09-14 13:40:14, 2021-09-14 13:40:14))

my livewire view look like this

  <form wire:submit.prevent="save" enctype="multipart/form-data">
          
              @foreach ($students as $student)
                <div class="form-group">
                  <label for="">Full Name : </label>
                  <label> <strong>{{ $student->FullName }}</strong></label>
                </div>
                <div class="form-group">
                  <input type="text"  class="form-control"  value="{{ $student->id }}"wire:model.defer="st_id">
                </div>
                <div class="form-group">
                  <label for="">Student ID : </label>
                  <label> <strong>{{ $student->student_id }}</strong></label>
                  <input type="text"  class="form-control"  value="{{ $student->student_id }}"wire:model.defer="student_id" readonly>
                  <span class="text-danger">@error('student_id')  {{ $message }} @enderror</span>
                </div>
              @endforeach
              <hr>

script

<script>
    window.addEventListener('OpenPaymentModal',function(){
      $('.addpayment').find('span').html('');
      $('.addpayment').find('form')[0].reset();
      $('.addpayment').modal('show');
  });

<div class="row">
        @foreach ($courses as $course)
        <div class="col-md-4 col-sm-6">
            <div class="card-body">
                <div class="card mt-3 shadow">
                    <div class="card-header">{{ $course->Name }}</div>
                    <img class="card-img-top center" src="{{ asset('assets/img/about.jpg') }}" alt="Card image cap">
                    <div class="card-body">
                        <hr>
                        <button class="btn btn-warning" wire:click="OpenPaymentModal({{ $course->id}})">Click here to Pay</button>
                    </div>
                </div>
            </div>
        </div>
        @endforeach

controller

namespace App\Http\Livewire;

use Livewire\Component;
use Illuminate\Support\Facades\Auth;
use App\Models\course;
use App\Models\Student;
use App\Models\Payment;

use Livewire\WithFileUploads;

class StudPayment extends Component
{
    use WithFileUploads;

    public $photo,$st_id,$student_id;
    public $course;
   

    public function render()
    {
        $courses = Auth::guard('student')->user()->courses()->get();
        $students = Auth::guard('student')->user()->get();
        return view('livewire.stud-payment',[
            'courses'=>$courses,
            'students'=>$students
        ]);
    }

    public function OpenPaymentModal($id)
    {
        $info = Student::find($id);
        $this->course = $info->id;
        $this->dispatchBrowserEvent('OpenPaymentModal',[
            'id'=>$id
        ]);
        
    }

    public function save()
    {
        $this->validate([
            'course'=>'required',
            'photo'=>'image|max:1024',
            
        ]);
        

        $payment = new Payment();

        $payment->image_path = $this->photo->store('images','public');
        $payment->student_id = $this->student_id;
        $payment->course_id = $this->course;
        $payment->st_id = $this->st_id;
        $save = $payment->save();

        if($save){
            $this->dispatchBrowserEvent('ClosepaymenttModal');
        }
    }
}