Laravel - Store data with a Polymorphic relation

I'm new to Laravel and would I'm confused on how to properly store a data from a form with a polymorphic relation. I have three models.

Salary Grade Model

public function salaryallowance() {
    return $this->morphOne('App\SalaryAllowanceModel', 'salary');
}

Salary Matrix Model

    public function salaryallowance() {
    return $this->morphOne('App\SalaryAllowanceModel', 'salary');
}

Salary Allowance Model

    public function salary() {
    return $this->morphTo();
}

Salary Allowance Controller

  public function create()
  {
        $getSalaryAllowanceWithSalary = SalaryAllowanceModel::all();
        $getPosition = PositionModel::pluck('position_name', 'id');
        $getSalaryGrade = SalaryGradeModel::pluck('salary_grade', 'id');
    return view('SalaryAllowanceView.add', compact('getSalaryAllowanceWithSalary', 'getPosition', 'getSalaryGrade'));
  }


    public function store(Request $request) {
    SalaryAllowanceModel::create ([
            'salary_id' => $request->position,
            'salary_type' => get_class($request),
            'salary_allowance' => $request->allowance,  
            'salary_transportation' => $request->transportation,
            'salary_medical' => $request->medical,
            'salary_education' => $request->education,
            'salary_housing' => $request->housing,
            'salary_clothing' => $request->clothing
        ]);
        return back();
       }

Blade.php

 <div class="form-group col-md-12 row">
                    <label class="col-md-4 col-form-label text-md-right">{{ __('Position') }}</label>
                    <div class="col-md-6">
                        <select name="position" class="form-control">
                            <option value="0">Please Select Position</option>
                            @foreach ($getPosition as $id => $position)
                            <option value="{{$id}}">{{ $position }}</option>
                            @endforeach
                        </select>
                    </div>
                </div>

                <div class="form-group col-md-12 row">
                    <label class="col-md-4 col-form-label text-md-right">{{ __('Allowance') }}</label>
                    <div class="col-md-6">
                        <input type="text" class="form-control" name="allowance">
                    </div>
                </div>

I'm having a hard time to do a syntax for storing the data. I want the ID to be based on the selected drop-down.

EDIT: I have tried using this approach but I'm getting the wrong ID to be saved in the column salary_id of Salary Allowance table. The model is properly saved in the salary_type.

 public function store(Request $request)
  {
    $post = SectionModel::find(2);
    $comment = new SalaryAllowanceModel;
    $comment->salary_allowance = $request->allowance;
    $comment->salary_id = $request->position;
    $post->salaryallowance()->save($comment);

        return back();
  }

EDIT 2: SOLVED Finally I have found the solution. Posting answer for people who have the same problem

 public function store(Request $request)
  {
      $post = SalaryMatrixModel::find($request->position);  
      $comment = new SalaryAllowanceModel;
      $comment->salary_allowance = $request->allowance;
      $comment->salary_id = $request->position;
      $post->salaryallowance()->save($comment);
    }