Index.cshtml

@{
    ViewData["Title"] = "Home Page";
}

@model RichEditSubmit.Models.TestModel

<script src="~/node_modules/jszip/dist/jszip.min.js"></script>
<script src="~/node_modules/devextreme-dist/js/dx.all.js"></script>
<link href="~/node_modules/devextreme-dist/css/dx.common.css" rel="stylesheet" />
<link href="~/node_modules/devextreme-dist/css/dx.light.css" rel="stylesheet" />

@* If you export a document to PDF on the client side (the exportToPdf method) or enable client PDF printing mode,
    you should register the pdfkit library before control script registration.*@
<script src="~/node_modules/devexpress-richedit/dist/pdfkit.js"></script>

<script src="~/node_modules/devexpress-richedit/dist/dx.richedit.js"></script>
<link href="~/node_modules/devexpress-richedit/dist/dx.richedit.css" rel="stylesheet" />

<script>
    function onSubmitButtonClick() {
        richEdit.saveDocument();
        return false;
    }
    function onSaving(s, e) {
        Base64.value = e.base64;
        Format.value = e.format;
        e.handled = true;
        $('form').submit();
    }
</script>

<form asp-controller="Home" asp-action="Index">
    @Html.HiddenFor(m => m.ID)
    @Html.HiddenFor(m => m.Base64)
    @Html.HiddenFor(m => m.Format)
    @Html.TextAreaFor(m => m.Description, new { style= "width: 100%" })
    <button onclick="return onSubmitButtonClick()">Submit</button>
    @Html.DevExpress().RichEdit("richEdit").Open(() => { return Convert.FromBase64String(Model.Base64); }, (DevExpress.AspNetCore.RichEdit.DocumentFormat)Model.Format).OnSaving("onSaving")
</form>