Disavow backlinks in ahrefs - how?

Ahrefs doesn't actually disavow anything. It can help you build the list and export it as a disavow file, but the file only does something once you upload it to Google's disavow tool yourself. Bigger point though: on a brand new site with almost no links, don't. Google ignores most spammy links on its own these days and the disavow tool is really meant for a manual action or a link scheme you were part of. You're more likely to throw away a link that was helping.