Yes, you sure can but it's a bit tricky. You need to have a knowledge of HTML selectors.
Choose 'Custom' from 'Countdown position' selectbox and you'll see a new field (Custom selector element) just below it. This is where you'd put your custom html selector.
For eg. if you want to put it on product page's add to cart or buy button, in most cases the custom selector is:
form.product-form
It's just an example, It may or may not work in your case. If it doesn't, Kindly contact us on live chat or create a ticket.
OR
you can put the following code in your theme where you want to render the timer:
<span class="sellify-ucd-timer-widget timer-001"></span>
and then in 'Custom selector element' put following:
.sellify-ucd-timer-widget.timer-001
You can change 'timer-001' to any number you want like 'timer-100', but make sure you do it in both places (your theme and custom selector element)