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 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();
       }

I'm having a hard time to do a syntax for storing the data.

via Chebli Mohamed