GiveServicePrincipalAccess.ps1

#Depending if you are already connected execute this first part
Connect-AzureAD

#Get the Service Principal that was created when you created a Managed Identity from your Web App
$MyMI = Get-AzureADServicePrincipal -ObjectId <ID(GUID) of your managed identity shown in the Azure portal>

#Get the Office 365 SharePoint Online API
$SharePointAPI = Get-AzureADServicePrincipal -SearchString “Office 365 SharePoint”

#Select the User.ReadWrite.All App-Only Permissions. If you want another App-Only permission then search for another value
$UserReadWrite = $SharePointAPI.AppRoles | where Value -like 'Sites.FullControl.All'

#Add the User.ReadWrite.All permissions from the Office 365 SharePoint Online API 
New-AzureADServiceAppRoleAssignment -Id $UserReadWrite.Id -ObjectId $MyMI.ObjectId -PrincipalId $MyMI.ObjectId -ResourceId $SharePointAPI.ObjectId