Create a Success Page on Form Submission

<div class="container">
    <div class="row">
        <div class="col">
            %%[IF RequestParameter("submitted") == 'true' THEN
            /* add in InsertDE() or Entry Event */]%%
            <p>Success Page</p>
            %%[ELSE]%%
            <form action="%%=RequestParameter('PAGEURL')=%%" method="post">
                <div>
                    <label for="firstname">First name:</label>
                    <input type="text" id="firstname" name="firstname">
                </div>
                <div>
                    <label for="lastname">Last name:</label>
                    <input type="text" id="lastname" name="lastname">
                </div>
                    <input name="submitted" type="hidden" value="true">
                    <button class="button" type="submit" value="Submit">Submit</button>
            </form>
            %%[ENDIF]%%
        </div>
    </div>
</div>