Interesting use case... Seems like you need some JavaScript to do this.
The following seems to do the job – paste it into the Footer HTML of the registration form. You'll need to make an obvious change (change the @mycompany.com string to whatever your Initial Value is). Works OK on Firefox and Chrome, I did not test it on Internet Explorer:
<script type="text/javascript">{literal}
(function () {
var f = document.forms[0].sponsor_email;
f.onfocus = function () {
if (f.value == "@mycompany.com") {
window.setTimeout(function () {
if (f.createTextRange) {
var range = f.createTextRange();
range.collapse(true);
range.select();
} else if (f.setSelectionRange) {
f.setSelectionRange(0, 0);
} else if (typeof f.selectionStart != "undefined") {
f.selectionStart = f.selectionEnd = 0;
}
},1);
}
};
})();
{/literal}</script>