public final HttpClientBuilder disableDefaultUserAgent() {
        this.defaultUserAgentDisabled = true;
        return this;
    }
Disables the default user agent set by this builder if none has been provided by the user.

@return this instance.
@since 4.5.7
httpclient5/src/main/java/org/apache/hc/client5/http/impl/classic/HttpClientBuilder.java