How to add value in Array using textbox as input with Add click button method

Following is What I am trying to acquire using powershell.

1. Adding User names i.e First Names in an array using Windows form methods
2. I am using textbox as input for First name
3. To assist each input I am using click button.

$usernames = @()
$usernames += [pscustomobject]@{
Firstnames=$firstname  
}

$form = New-object System.Windows.Forms.Form
$form.Location = "500, 500"
$form.Text = "Add User names"

$textbox = New-object System.Windows.Forms.Textbox 
$textbox.Location = "55, 130"
$textbox.size = "114, 40"
$textbox.text = ""

$button = New-object System.Windows.Forms.Button
$button.Location = "55, 150"
$button.size = "155, 23"
$button.text = Add User name


$button.Add_Click = {($firstname = $textbox.text)}

$form.controls.Add($button.Add_Click)
$form.controls.Add($textbox)
$form.ShowDialog() | Out-null

It's not entirely clear what your question is but presumably you want the .Text values to be added to a list when clicking the button, in which case, you can approach as below. Note, you have to use a List<> or a collection type that allows adding, an array (@() with += is invalid).

Add-Type -AssemblyName System.Windows.Forms

# here is where users get added when clicking the button
$addedusers = [System.Collections.Generic.List[string]]::new()

$form = [System.Windows.Forms.Form]@{
    Location = '500, 500'
    Text     = 'Add User names'
}

$textbox = [System.Windows.Forms.Textbox]@{
    Location = '55, 130'
    Size     = '114, 40'
}

# this handler enables and disables the button based on the text value (if all blank = disabled)
$textbox.Add_TextChanged({
    if ([string]::IsNullOrWhiteSpace($this.Text)) {
        $button.Enabled = $false
        return
    }

    $button.Enabled = $true
})

# start with the button in disabled state
$button = [System.Windows.Forms.Button]@{
    Location = '55, 150'
    Size     = '155, 23'
    Text     = 'Add User name'
    Enabled  = $false
}

# handler to add the text value to the list<string>
$button.Add_Click({
    $addedusers.Add($textbox.Text)
    $textbox.Text = ''
})

$form.Controls.AddRange(@($textbox, $button))
$form.ShowDialog() | Out-Null

# show the added users when form is closed
$addedusers