ko async validation plugin - ajax

Question

[UPDATED]
I am not sure how to add a custom (html) message to my async knockout validator.
Below is a simple form. You can see that there is already some isModified and isValid validation in-place, but it is not async. (I am not sure if this complicates or simplifies the solution.)
<form
method="post"
action="Registration()"
novalidate
data-bind="submit: onRegister">
<div
class="input-wrapper email"
data-bind="
validationOptions: {
messageTemplate: 'emailCollisionTemplate'
},
validationElement: EmailAddress,
css: { 'validation-success': EmailAddress.isModified() && EmailAddress.isValid() }">
<input
type="email"
id="registerModalEmail"
data-bind="
value: EmailAddress,
ariaInvalid: EmailAddress"
aria-describedby="email-address-error"
required />
</div>
<div
class="validation-error-message"
id="email-address-error"
data-bind="validationMessage: EmailAddress">
</div>
</form>
<script type="text/html" id="emailCollisionTemplate">
<span>This email is already registered. Please sign in.
</span>
</script>
And then some js:
self.EmailAddress = ko.observable().extend({
required: {
params: true,
message: 'required'
)
},
pattern: {
params: '#',
message: 'match'
)
},
validation: {
async: true,
validator: function (val, parms, callback) {
$.ajax({
url: 'myUrl/isEmailAllowed',
data: { 'EmailAddress': val },
type: 'POST',
success: function (results) {
if (results) {
callback(true);
}
},
error: function (xhr) {
callback(false);
}
});
}
}
});
(isModified and isValid is handled elsewhere, so I have not included it.)
How do I configure the messageTemplate to show? I've looked at examples, but none of them seem to match what I'm doing here. I've put the validationOptions into the data-bind like it seems to say, but that's not working.
I should point out two confounding aspects here:
There are other validators on this same field (i.e.required and pattern)
There are other fields in this form with validation. (The configuration object says the messageTemplate should be 'id of the script that you want to use for all your validation messages'. I only want it for this one field.)