using NetFwTypeLib; // Located in FirewallAPI.dll
...
INetFwRule firewallRule = (INetFwRule)Activator.CreateInstance(
    Type.GetTypeFromProgID("HNetCfg.FWRule"));
firewallRule.Action = NET_FW_ACTION_.NET_FW_ACTION_BLOCK;
firewallRule.Description = "Used to block all internet access.";
firewallRule.Direction = NET_FW_RULE_DIRECTION_.NET_FW_RULE_DIR_OUT;
firewallRule.Enabled = true;
firewallRule.InterfaceTypes = "All";
firewallRule.Name = "Block Internet";

INetFwPolicy2 firewallPolicy = (INetFwPolicy2)Activator.CreateInstance(
    Type.GetTypeFromProgID("HNetCfg.FwPolicy2"));
firewallPolicy.Rules.Add(firewallRule);
-------------
 using System.Management.Automation;
...
private void OpenPort(int port)
{
    var powershell = PowerShell.Create();
    var psCommand = $"New-NetFirewallRule -DisplayName \"<rule description>\" -Direction Inbound -LocalPort {port} -Protocol TCP -Action Allow";
    powershell.Commands.AddScript(psCommand);
    powershell.Invoke();
}