Le blog technique

# The app for which consent is being granted.
$clientAppId = "de8bc8b5-d9f9-48b1-a8ad-b748da725064" # Microsoft Graph Explorer

# The API to which access will be granted. Microsoft Graph Explorer makes API 
# requests to the Microsoft Graph API, so we'll use that here.
$resourceAppId = "00000003-0000-0000-c000-000000000000" # Microsoft Graph API

# The permissions to grant. Here we're including "openid", "profile", "User.Read"
# and "offline_access" (for basic sign-in), as well as "User.ReadBasic.All" (for 
# reading other users' basic profile).
$permissions = @("openid", "profile", "offline_access", "User.Read", "User.ReadBasic.All")

# The user on behalf of whom access will be granted. The app will be able to access 
# the API on behalf of this user.
$userUpnOrId = "user@example.com"

# Step 0. Connect to Microsoft Graph PowerShell. We need User.ReadBasic.All to get
#    users' IDs, Application.ReadWrite.All to list and create service principals, 
#    DelegatedPermissionGrant.ReadWrite.All to create delegated permission grants, 
#    and AppRoleAssignment.ReadWrite.All to assign an app role.
#    WARNING: These are high-privilege permissions!
Connect-MgGraph -Scopes ("User.ReadBasic.All Application.ReadWrite.All " + "DelegatedPermissionGrant.ReadWrite.All " + "AppRoleAssignment.ReadWrite.All")

# Step 1. Check if a service principal exists for the client application. 
#     If one doesn't exist, create it.
$clientSp = Get-MgServicePrincipal -Filter "appId eq '$($clientAppId)'"
if (-not $clientSp) {
   $clientSp = New-MgServicePrincipal -AppId $clientAppId
}

# Step 2. Create a delegated permission that grants the client app access to the
#     API, on behalf of the user. (This example assumes that an existing delegated 
#     permission grant does not already exist, in which case it would be necessary 
#     to update the existing grant, rather than create a new one.)
$user = Get-MgUser -UserId $userUpnOrId
$resourceSp = Get-MgServicePrincipal -Filter "appId eq '$($resourceAppId)'"
$scopeToGrant = $permissions -join " "
$grant = New-MgOauth2PermissionGrant -ResourceId $resourceSp.Id -Scope $scopeToGrant -ClientId $clientSp.Id -ConsentType "Principal" -PrincipalId $user.Id

# Step 3. Assign the app to the user. This ensures that the user can sign in if assignment
#     is required, and ensures that the app shows up under the user's My Apps portal.
if ($clientSp.AppRoles | ? { $_.AllowedMemberTypes -contains "User" }) {
    Write-Warning ("A default app role assignment cannot be created because the " + "client application exposes user-assignable app roles. You must " + "assign the user a specific app role for the app to be listed " + "in the user's My Apps portal.")
} else {
    # The app role ID 00000000-0000-0000-0000-000000000000 is the default app role
    # indicating that the app is assigned to the user, but not for any specific 
    # app role.
    $assignment = New-MgServicePrincipalAppRoleAssignedTo -ServicePrincipalId $clientSp.Id -ResourceId $clientSp.Id -PrincipalId $user.Id -AppRoleId "00000000-0000-0000-0000-000000000000"
}