You created the element, so append it. innerHTML
is only for when you have an HTML string.
let inputCategory = document.querySelector("#modal_form_product select[name='category']");
let option = document.createElement("option");
option.innerText = "5555";
option.value = "5555";
inputCategory.appendChild(option);
or
let inputCategory = document.querySelector("#modal_form_product select[name='category']");
inputCategory.innerHTML += `<option value=5555>5555</option>`;
or
let inputCategory = document.querySelector("#modal_form_product select[name='category']");
inputCategory.insertAdjacentHTML('beforeend', `<option value=5555>5555</option>`);
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