When you're opening your modal, you can reset it based on your requirements. Below are the js functions that I use in my forms when the modal pops up. 
So, when the modal popup you need to pass your input field in this function. It'll preset the form value when the form has already submitted

function (input) {
    var parentForm = input.closest('form');
    if (parentForm === null || !parentForm.classList.contains('EPiServerForms')) {
        input.value = '';
    }
    else {
        var formMainBody = parentForm.querySelector(".Form__MainBody");
        if (formMainBody && formMainBody.style.display === "none") {
            formMainBody.removeAttribute("style");
            var formStatusMessage = parentForm.querySelector(".Form__Status__Message");
            if (formStatusMessage) {
                formStatusMessage.innerHTML = ''
                formStatusMessage.classList.remove("Form__Success__Message", "Form__Warning__Message");
                formStatusMessage.classList.add("hide");
            }
            parentForm.reset();
        }
    }
}