To manually set the HTTP version for your HTTP monitoring requests, add the corresponding code into the Prepare Script filed under the Scripts tab of your HTTP target configuration screen:

HTTP/1.1
//%DotcomWebComment% HttpClient.HttpVersion = System.Net.HttpVersion.Version11;
//%DotcomWebComment% HttpClient.VersionPolicy = System.Net.Http.HttpVersionPolicy.RequestVersionExact;
HTTP/2.0
//%DotcomWebComment% HttpClient.HttpVersion = System.Net.HttpVersion.Version20;
//%DotcomWebComment% HttpClient.VersionPolicy = System.Net.Http.HttpVersionPolicy.RequestVersionExact;
HTTP/3.0
//%DotcomWebComment% HttpClient.HttpVersion = System.Net.HttpVersion.Version30;
//%DotcomWebComment% HttpClient.VersionPolicy = System.Net.Http.HttpVersionPolicy.RequestVersionExact;