I believe you can try to provide the NTLM credentials with the basic authentication option. This way is possible using a browser monitor.
Not sure if it would be different for HTTP monitors. @Philipp K. can you help with this?
Currently, there is no OOTB support for NTLM authentication for HTTP Monitors (only for Browser monitor). You can make a multi request and use pre-script and post-script to do the authentication but is going to require some development.
The support for NTLM in HTTP monitors is already on the road map, but there is no ETA on when this feature will be available.
Followup to the question about Kerberos - I also have monitoring targets where NTLM is an option. It does seem easier to implement than Kerberos due to only having client <> server communication, but reading this material makes think it would still be quite a project to implement actually: