Hi all.

I found out.

ruleType is always MemberRole and ruleValue is the name of your Member Group name.

// Umbraco Services
var publicAccessService = ApplicationContext.Current.Services.PublicAccessService;
var contentService = ApplicationContext.Current.Services.ContentService;

// Get IContent that should be protectet.
var content = contentService.GetById(1000);

// Get the current role for the content
var entry = publicAccessService.GetEntryForContent(content);

// Update entry
entry.LoginNodeId = 1500;
entry.NoAccessNodeId = 1660;
entry.AddRule("MemberRole", "NewCustomRole");
publicAccessService.Save(entry);

// If entry does not exists, create one this way.
var loginPage = ApplicationContext.Current.Services.ContentService.GetById(1001);
var noAccessPage = ApplicationContext.Current.Services.ContentService.GetById(1002);
var entry = new PublicAccessEntry(content, loginPage, noAccessPage, new List<PublicAccessRule>());
publicAccessService.Save(entry);

// After the new entry is saved, you can add new rule this way.
// Or if you just wanna add rules and not LoginNodeId and NoAccessNodeId. 
// If entry is null. This won't work.
var roleBasedProtectionAttempt = publicAccessService.AddRule(content, "MemberRole", "MyCustomRole");