NSMutableURLRequest *someURLReq = ...
NSString *auth = ...
[someURLReq setValue:auth forHTTPHeaderField:@"Authorization"];

NSString *username = ...
NSString *password = ...
NSString *combo = [NSString stringWithFormat:@"%@:%@", username, password];
NSData *comboData = [NSData dataWithBytes:[combo UTF8String] length:combo.length];
NSString *auth = [NSString stringWithFormat:@"Basic %@", [comboData encodeBase64]];