How Do I Make Input Mandatory And Start With A 99 Or 98

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jQuery

validation

forms

I currently have the code below which is forcing numerical input but how do i ensure there is some input and that is starts with a 98 or 99.

<script>
<!--
       function isNumberKey(evt)
       {
          var charCode = (evt.which) ? evt.which : evt.keyCode;
          if (charCode != 46 && charCode > 31
            && (charCode < 48 || charCode > 57))
             return false;

          return true;
       }
       //-->
</script>

With the form being

<form method="post" action="insurance/">
            <span class="input input--fumi">
                    <input class="input__field input__field--fumi" onkeypress="return isNumberKey(event)" type="text" maxlength="11" id="input-25" />
                    <label class="input__label input__label--fumi" for="input-25">
                        <i class="fa fa-fw fa-credit-card icon icon--fumi"></i>
                        <span class="input__label-content input__label-content--fumi">Membership Number</span>
                    </label>
                </span>
            <button type="submit" class="btn-next">Next</button>

Thanks in advance for your help