Get Multiselect value from toOptionArray() Magento 2

I have created system.xml with the following configuration –

<group id="invoice_toggle_config" translate="label" showInDefault="1" showInWebsite="1"
               showInStore="1">
            <label>Invoice/Billing History Toggle Config</label>
            <field id="invoice_search_by_options" translate="label" type="multiselect" sortOrder="31"
                   showInDefault="1"
                   showInWebsite="1"
                   showInStore="1">
                <label>Search By Invoice Options</label>
                <source_model>XYZOrderHistoryModelConfigSourceSearchByInvoiceOptions</source_model>
            </field>
            <field id="invoice_filter_by_options" translate="label" type="multiselect" sortOrder="31"
                   showInDefault="1"
                   showInWebsite="1"
                   showInStore="1">
                <label>Filter By Invoice Options</label>
                <source_model>XYZOrderHistoryModelConfigSourceFilterByInvoiceOptions</source_model>
            </field>
        </group>

And the corresponding files SearchByInvoiceOptions & FilterByInvoiceOptions consists of following toOptionArray()-

SearchByInvoiceOptions.php

public function toOptionArray(): array
{
    return [
        [
            'label' => __('Date'),
            'value' => 'filter_by_date'
        ],
        [
            'label' => __('Status'),
            'value' => 'filter_by_status'
        ],
        [
            'label' => __('Invoice Total'),
            'value' => 'invoice_total'
        ]
    ];
}

FilterByInvoiceOptions.php

public function toOptionArray(): array
{
    return [
        [
            'label' => __('Invoice #'),
            'value' => 'invoice_number'
        ],
        [
            'label' => __('PO #'),
            'value' => 'po_number'
        ], [
            'label' => __('Order #'),
            'value' => 'order_number'
        ]
    ];
}

Now, I want the selected values via schema.graphQL i.e. when I hit the API at postman, I should get the response as the values that are selected. Here, suppose at the backend these below 3 labels are selected only. Then i should get their values like below-

    "invoice": {
  invoice_search_by_options: {"po_number","order_number"},
  invoice_filter_by_options: {"invoice_total"},

}

Can anyone guide me at the graphql part? how that can be achieved