Microsoft does not provide a way to modify the cipher suite in the code of the .net Framework 4.5, maybe you should consider other ways to do this.
The selection and negotiation of cipher suites in the .NET Framework is handled by the operating system's SSL/TLS library (Windows Schannel in this case), and we can override the default cipher suites in Schannel through group policies and registry keys.
Manage Transport Layer Security (TLS)
In addition, I found this document:
Prioritizing Schannel Cipher Suites
It seems that you can perform certain operations on the cipher suite in C++, but this requires you to post a new question and ask a C++ expert for confirmation.
The following link also mentions a way to modify the cipher suite, you can also refer to it.
Need to set cipher suite at the code in C#
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