Siesta selector by custom tags

I want to ask if in siesta possible to write selector by custom tag:

for example I have component:

<div class="x-window x-layer x-window-default x-border-box" id="window-1009" tabindex="-1" data-id="loginWnd" style="width: 400px; height: 150px; right: auto; left: 600px; top: 483px; z-index: 19000;">

which from console I can select using css selector: document.querySelector('a[data-id="loginBtn"]')

but if I write in siesta test: t.chain({click: '>> a[data-id="loginBtn"]'}, no component is find.

How the selector should look like to find component?