How do I write a function that returns a string and set as one of parameter in cmdlet in powershell

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C#

.NET

PowerShell

I have zero knowledge of widows powershell. I have situation where i need to create parameter and function(a wrapper). Function when invoked return string and set the value to one of the parameter. I don't even know where do i write powershell script and i think there is ISE where i can write but how to build them get the bat file?

My situation is something like this.

Command is ExportBill.exe which takes three arguements : filename, customerType and Id

function GenerateFileName(string fName)
{
    return String.Format("{0}{1}",fName.ToUpper().Substring(0, 3),DateTime.Now.ToString("yyyyMMddHHmmss"));
}

ExportBill.exe -fileName GenerateFileName() -customerType "WholeSeller -@Id query_parameters

How do i do it in Powershell?

What I have tried:

I have never write a script in powershell. I have no idea.