Add UserCustomAction using Powershell CSOM in Office 365

AddUserCustom.ps1

Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll" 
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Publishing.dll"

# Authenticate with the SharePoint site. 
# 
$siteUrl = Read-Host -Prompt "Enter site url" 
$username = Read-Host -Prompt "Enter Username" 
$password = Read-Host -Prompt "Enter password" -AsSecureString 
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl) 

# SharePoint Online
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password) 
$ctx.Credentials = $credentials

$rootWeb = $ctx.Web
$ctx.Load($rootWeb) 
$caColl = $rootWeb.get_userCustomActions()
$ctx.Load($caColl) 
$ctx.ExecuteQuery() 

$cUIExtn = "<CommandUIExtension><CommandUIDefinitions><CommandUIDefinition Location=""Ribbon.List.Share.Controls._children"">
<Button Id=""Ribbon.List.Share.ExportToExcelButton"" Alt=""Export to Excel"" Sequence=""11"" Command=""Invoke_ExportToExcelButtonRequest"" LabelText=""Export to Excel"" TemplateAlias=""o1"" Image32by32=""_layouts/15/images/placeholder32x32.png"" Image16by16=""_layouts/15/images/placeholder16x16.png"" />
</CommandUIDefinition></CommandUIDefinitions><CommandUIHandlers>
<CommandUIHandler Command=""Invoke_ExportToExcelButtonRequest"" CommandAction=""javascript: function exportSchema() { var field = function (Name, Type, ID, rFieldName) {this.DisplayName = Name; this.FieldType = Type; this.ID = ID; this.RealFieldName = rFieldName; };
var listFields = []; for (var i=0; i&lt;ctx.ListSchema.Field.length; i++) {listFields.push(new field(ctx.ListSchema.Field[i].DisplayName, ctx.ListSchema.Field[i].FieldType, ctx.ListSchema.Field[i].ID, ctx.ListSchema.Field[i].RealFieldName));} 
var form = document.createElement('form'); form.setAttribute('method', 'post'); var hField = document.createElement('input'); hField.setAttribute('type', 'hidden');
hField.setAttribute('name', 'ListFields'); hField.setAttribute('value', JSON.stringify(listFields)); form.appendChild(hField);
var hViewID = document.createElement('input'); hViewID.setAttribute('type', 'hidden'); hViewID.setAttribute('name', 'ViewID'); hViewID.setAttribute('value', ctx.view.replace('{','').replace('}','')); form.appendChild(hViewID);
var hListID = document.createElement('input'); hListID.setAttribute('type', 'hidden'); hListID.setAttribute('name', 'ListID'); hListID.setAttribute('value', ctx.listName.replace('{','').replace('}','')); form.appendChild(hListID);
var hlistName = document.createElement('input'); hlistName.setAttribute('type', 'hidden'); hlistName.setAttribute('name', 'ListName'); hlistName.setAttribute('value', ctx.ListTitle); form.appendChild(hlistName);
var hItemIDs = document.createElement('input'); hItemIDs.setAttribute('type', 'hidden'); hItemIDs.setAttribute('name', 'ItemIDs'); hItemIDs.setAttribute('value', '{SelectedItemId}'); form.appendChild(hItemIDs);
form.setAttribute('action', 'http://localhost:3000/export');
document.body.appendChild(form); form.submit(); } exportSchema();""          
EnabledScript=""javascript: function exporttoexcelenable() { return (true);} exporttoexcelenable();""/></CommandUIHandlers></CommandUIExtension>"

$newUCA = $caColl.Add()
$newUCA.set_registrationId("100");
$newUCA.set_registrationType("List");
$newUCA.set_location('CommandUI.Ribbon');
$newUCA.set_title('Invoke ExportToExcel Action');
$newUCA.set_commandUIExtension($cUIExtn)

$newUCA.update()
$ctx.ExecuteQuery()