Product Quantity increase by Quantity+Quantity

[Zhado]

Hello Everyone! Please help me to solve this issue.

I have implemented an app-MultiVendor Marketplace to my store. Now all the products MOQ (minimum order quantity) is auto stored to my input field with this class="wk_qty_selector" 

Now I want to display double quantity when increase button is pressed (+)

For example MOQ is 5, so when increased- input value would be  5, 10, 15, 20... when decreased 20,15,10,5 accordingly 

Actually, this line:  <div class="qty_value"></div>    displays the initial MOQ from seller to the website but I can not take from there a value to assign in for input step.

I have tried to get the value from class as: var elements = document.getElementsByClassName('qty_value');

but it did not work.

Here is my code for input: 

<quantity-input class="quantity">
<button class="quantity__button no-js-hidden" name="minus" type="button">
<span class="visually-hidden">{{ 'products.product.quantity.decrease' | t: product: product.title | escape }}</span>
{% render 'icon-minus' %}
</button>

<input class="wk_qty_selector"
style="pointer-events:none; color:currentColor; font-size:1.4rem; font-weight:500; opacity:0.85; text-align:center; background-color:transparent; border:0; padding: 0 0.5rem; width: 100%; flex-grow: 1; -webkit-appearance:none; appearance: none"
type="number"
name="quantity"
id="Quantity-{{ section.id }}"
min="1"
value="1"
form="{{ product_form_id }}"
readonly>

<button class="quantity__button no-js-hidden" name="plus" type="button">
<span class="visually-hidden">{{ 'products.product.quantity.increase' | t: product: product.title | escape }}</span>
{% render 'icon-plus' %}
</button>
</quantity-input>