How can I set TextBoxFor to be hidden by default

Not sure what you mean by default, but you could add the style attribute:

<%= Html.TextBoxFor(m => m.SomeProperty, new { style = "display: none;" }) %>

or:

<%= Html.TextBoxFor(m => m.SomeProperty, new { @class = "hidden" }) %>

and in your CSS file:

.hidden {
    display: none;
}

Html.TextBoxFor(m => m.SomeProperty, new { @readonly = "readonly", @hidden="true"})

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