Link to an entry via Entries field type
Even if you set the limit to 1, Craft always returns an array of entries. So you need to add first() to access the first (and only) entry.

{{ entry.buttonTargets.first() }}


It you don't add a property, this returns the entry's title by default. So you additionally need to add url to output the URL of that entry.

{{ entry.buttonTargets.first().url }}