your script

<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">

<g:ui_form>

<div class="row">
<div >
<span style="padding:16px;font-weight:bold;">What were you attempting to do when the performance issue occurred?</span>
</div>
</div>
<div class="row">
<div class="form-horizontal">
<div class="form-group" style="margin-left:16px !important;margin-right:16px !important">
<textarea id="comments" name="comments"></textarea>
</div>
</div>
</div>
<div class="modal-footer">
<span class="pull-right">

<button class="btn btn-primary" id="ok_button" onclick="actionOK()" style="min-width: 5em;" title="" type="submit">
Send Email
</button>
</span>
</div>
<input type="hidden" id="kbSysID" name="kbSysID" value="${sysparm_sysID}"/>
</g:ui_form>

</j:jelly>

function actionOK() {

// 	alert("entered");
	var value = gel('comments').value;
// 	alert(value);
	if (value != '') {
		alert("comments filled");
		return true;

	} else {
		alert("please fill");
		return false;
	}
}