Note that this requires the premium version of Lansweeper.
Our Lansweeper is installed in domain "Lansweeper"
We want to scan the untrusted domain "Untrusted"
First add both the Dns name and the Netbios name of the untrusted domain to the Domain scanning tab.
See this link: How to find the netbios name of your domain
Specify the credentials needed to connect to the untrusted domain.
Note that you might need to check "Don't try to authenticate username/password"
Make sure that the DNS settings on the Lansweeper server can resolve "Untrusteddomain.local"
You can do this by appending the DNS suffix and by specifying the correct DNS server.