Looks like it's the entry options DoNotNegotiateMultilink property that sets that value. Which, you'll need to bump up to the WINXP flavor to gain access to it. That also implies you'll only support Windows XP and later for your application.
using (RasPhoneBook pbk = new RasPhoneBook())
RasEntry entry = pbk.Entries["My Entry"];
entry.Options.DoNotNegotiateMultilink = false; // Sets the checkbox.
entry.Options.DoNotNegotiateMultilink = true; // Clears the checkbox.
For future reference, I've gone through the SDK and have identified which checkbox maps to which property in the remarks section of the property. Doing a search in the help documentation for the checkbox text should help you locate most of them quite