Having issues creating a random num/word generator...

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I've made one before but I lost it, and now im having issues making another one, I got close but seems like I cant figure it all out, im using console application C# VB.net
This is my code atm: https://pastebin.com/RwhiAmgL
Any ideas what I need to do? 
 

Code:

Edit: So I got this:   string str = generator.RandomString(26, false);
            Console.WriteLine($"Random string of 26 chars is {str + rand}");
            Which generates Letters plus number, but numbers are only at the end, when I need them all between.
For example:
I need it to look like this: 9WJLNN8MNDVJCFLQJ4W93YH6ZM:ZWN6QV9ZXG9YCMWAXXWP492DS9

So I need to make 26 length with colon then 26 more length, and make it loop so it creates multiple times, for example: 100k

Is there a better way to do this?

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