powershell mandatory parameter with default value shown

Here's a short example that might help:

    [CmdletBinding()]
    Param(
        $SqlServiceAccount = (Read-Host -prompt "SqlServiceAccount ($($env:computername + "_sa"))"),
        $SqlServiceAccountPwd = (Read-Host -prompt "SqlServiceAccountPwd")
    )
    if (!$SqlServiceAccount) { $SqlServiceAccount = $env:Computername + "_sa" }
    ...

By definition: mandatory parameters don't have default values. Even if you provide one, PowerShell will prompt for value unless specified when the command is called. There is however a 'hacky' way to get what you ask for. As variables (and as consequence - parameters) can have any name you wish, it's enough to define command with parameters that match the prompt you would like to see:

function foo {
    param (
        [Parameter(Mandatory = $true)]
        [Alias('Parameter1')]
        [AllowNull()]
        ${Parameter1[default value]},
        [Parameter(Mandatory = $true)]
        [Alias('Parameter2')]
        [AllowNull()]
        ${Parameter2[1234]}
    )
    $Parameter1 = 
        if (${Parameter1[default value]}) {
            ${Parameter1[default value]}
        } else {
            'default value'
        }
    $Parameter2 = 
        if (${Parameter2[1234]}) {
            ${Parameter2[1234]}
        } else {
            1234
        }
    [PSCustomObject]@{
        Parameter1 = $Parameter1
        Parameter2 = $Parameter2
    }
}

When called w/o parameters, function will present user with prompt that match parameter names. When called with -Parameter1 notDefaultValue and/or with -Parameter2 7, aliases will kick in and assign passed value to the selected parameter. As variables named like that are no fun to work with - it makes sense to assign value (default or passed by the user) to variable that matches our alias/ fake parameter name.