Disable indexing of the preview website

What am I doing wrong when I try to ignore the indexing of the preview website with robots.txt like here https://www.gatsbyjs.com/plugins/gatsby-plugin-robots-txt/#netlify
the preview website/domain has the same robots content as the production.

Never mind. I figured that i was not even doing preview builds, but had to redirect the default subdomain to custom subdomain via toml file.