easiest example i could think of using a runspace

nonblockingform.ps1

using namespace System.Management.Automation.Runspaces
using namespace System.Windows.Forms
using namespace System.Drawing

Add-Type -AssemblyName System.Windows.Forms, System.Drawing

$form = [Form]@{ StartPosition = [FormStartPosition]::CenterScreen }

$iss = [initialsessionstate]::CreateDefault2()
$iss.Variables.Add([SessionStateVariableEntry]::new('form', $form, ''))
$rs = [runspacefactory]::CreateRunspace($iss)
$rs.Open()

$txtBox = [TextBox]@{
    Location = '10, 10'
    Size     = '200, 200'
    Font     = [Font]::new('Segoe UI', 15)
    ReadOnly = $true
    Name     = 'myTxtBox'
}

$btn = [Button]@{
    Location = '10, 50'
    Text     = 'Click Me!'
    Name     = 'myBtn'
}
$btn.Add_Click({
    $this.Enabled = $false
    $ps = [powershell]::Create().AddScript({
        $txtBox = $form.Controls.Find('myTxtBox', $false)[0]
        $txtBox.Clear()

        'hello world!'.GetEnumerator() | ForEach-Object {
            $txtBox.Text += $_
            Start-Sleep -Milliseconds 500
        }

        $form.Controls.Find('myBtn', $false)[0].Enabled = $true
    })
    $ps.Runspace = $rs
    $ps.BeginInvoke()
})

$form.Controls.AddRange(@($txtBox, $btn))
$form.ShowDialog()

if ($rs) { $rs.Dispose() }