[function] Create a function with optional call variables

Is there a way to create a parameter in a PowerShell function where you have to call it in order to have it considered?

An example given by commandlet (the bold being what I want to do):

Invoke-Command -computername Server01 -Scriptblock {...}

Here is an example of what I want to do with the function

Function DoStuff($computername, -arg2, -domain $domain)
    Test-parameter(-domain) if (-domain -eq $true) {
        use $domain
    }
    Else {
        $domain = "Domain1"
    }
    test-parameter($arg2) {
        if ($arg2 -eq $true) {
            Do something
        }
        else {
            Do the opposite
        }
    }

So in summary:

If "-arg2" is present, I want something to happen in the script. If "-Domain" is present and has an argument with it, I want that to be used rather then the set argument.

Not sure I understand the question correctly.

From what I gather, you want to be able to assign a value to Domain if it is null and also what to check if $args2 is supplied and according to the value, execute a certain code?

I changed the code to reassemble the assumptions made above.

Function DoStuff($computername, $arg2, $domain)
{
    if($domain -ne $null)
    {
        $domain = "Domain1"
    }

    if($arg2 -eq $null)
    {
    }
    else
    {
    }
}

DoStuff -computername "Test" -arg2 "" -domain "Domain2"
DoStuff -computername "Test" -arg2 "Test"  -domain ""
DoStuff -computername "Test" -domain "Domain2"
DoStuff -computername "Test" -arg2 "Domain2"

Did that help?

I don't think your question is very clear, this code assumes that if you're going to include the -domain parameter, it's always 'named' (i.e. dostuff computername arg2 -domain domain); this also makes the computername parameter mandatory.

Function DoStuff(){
    param(
        [Parameter(Mandatory=$true)][string]$computername,
        [Parameter(Mandatory=$false)][string]$arg2,
        [Parameter(Mandatory=$false)][string]$domain
    )
    if(!($domain)){
        $domain = 'domain1'
    }
    write-host $domain
    if($arg2){
        write-host "arg2 present... executing script block"
    }
    else{
        write-host "arg2 missing... exiting or whatever"
    }
}