Add class to svelte component

I created two components:

1. btn.svelte

<button class="btn" />

2. btnHold.svelte

<script>    import Btn from './btn.svelte';</script>

I'm trying to add a new class btn--hold to btn.svelte

<Btn class="{btn} btn--hold" />

I get an error on class.

Basically I would like in the end to have:

<button class="btn btn--hold" />

How can I add a class to an imported component?