<CommandUIHandler Command="Invoke_RibbonCustomAction1ButtonRequest"
                  CommandAction="javascript:alert('123');"/>

<CustomAction ScriptSrc="~appWebUrl/Scripts/HelloWorld.js"
              Location="ScriptLink" Sequence="1">

<CommandUIHandler Command="Invoke_RibbonCustomAction1ButtonRequest"
                  CommandAction="javascript:tryMe();"/>

<CommandUIExtension>
<CommandUIDefinitions>
    <CommandUIDefinition Location="Ribbon.Library.ViewFormat.Controls._children">
        <Button Id="Ribbon.Library.ViewFormat.About"
            Command="AboutButtonCommand"
            LabelText="About"
            Image32by32="{SiteUrl}/_layouts/15/1033/Images/formatmap32x32.png?rev=23"
            Image32by32Top="-273"
            Image32by32Left="-1"
            Description="About"
            TemplateAlias="o1" />
    </CommandUIDefinition>
</CommandUIDefinitions>
<CommandUIHandlers>
    <CommandUIHandler
        Command="AboutButtonCommand"
        CommandAction="javascript:aboutScript({SelectedItemId});"
        EnabledScript="javascript:onlyOneItemSelected();" />
</CommandUIHandlers>

function aboutScript(itemId) {
    alert("Hello user! You have selected item " + itemId);
}

function onlyOneItemSelected() {
    return (SP.ListOperation.Selection.getSelectedItems().length == 1)
}

Add-PnPFile -Path .AboutButtonScript.js -Folder "Scripts"

$ribbon = Get-Content .MyRibbon.xml
$ribbon = [string]$ribbon
Add-PnPCustomAction -Name "RibbonTester" -Title "RibbonTester" -Description "-" -Group "Tester" -Location "CommandUI.Ribbon" -CommandUIExtension $ribbon -RegistrationType ContentType -RegistrationId 0x0101

Add-PnPJavaScriptLink -Name "AboutButtonScript" -Url https://mytenant.sharepoint.com/sites/perm-tester/Scripts/AboutButtonScript.js -Scope Web