Hi
Below is an option that has worked for us.
It works using the Forms 2.0 API and JS.
After loading the form a callback function is called that will set the value of a text field then set the it to read only.
In this example it selects the MobilePhone field. But can be applied to any any form fields, by replacing id and value.
<script src="//app-sjo.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_1311"></form>
<script>
MktoForms2.loadForm("//app-sjo.marketo.com", "XXX-XXX-XXX", 1311, function(form){
form.setValues({ "MobilePhone": "999-999-999"});
document.getElementById('MobilePhone').setAttribute('readonly', 'true');
});
</script>