Creating a custom text field

[RNSL]

I am trying to create a custom text field on the product page.
I have tried entering this snippet of code on various places:

<label for="Letter">
      Letter
      <br>
      <input
        required
        placeholder="Letter"
        form="{{ product_form_id }}"
        type="text"
        id="Letter"
        name="properties[Letter]"
      >
    </label>

The field shows, but whenever I enter a value, the input field gets cleared the moment I click away and the "Buy it now" button disappears on me.
I am using the Dawn Theme.

Any idea why that would happen?

[RNSL]

By way of update: this happens even with apps from the app store that I've attempted to use in this regard. Same problem: the interface glitches out.