Summary

SharePoint related MS Graph API call requires {sitesid}, for example for the following API call we need {sitesId}

Create permission – Microsoft Graph v1.0 | Microsoft Docs

POST /sites/{sitesId}/permissions

Solution

Login to your SharePoint site. Go to Graph Explorer, https://aka.ms/GE, enter the following request call, assuming tenant as m365x162783 and site as Test1.

https://graph.microsoft.com/v1.0/sites/m365x162783.sharepoint.com:/sites/Test1?$select=id

You will get a response something like the following.

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#sites(id)/$entity",
    "id": "m365x162783.sharepoint.com,2e0ad342-19b2-4116-b6a5-2dc4867cb498,0efde304-9646-43d8-8eee-5b1a55ab17f1"
}

What you get back in the id is in this format:

{hostname},{spsite.id},{spweb.id}

Conclusion

This is a simple way to get this sites-id information for any future MS Graph API calls if you may want to explore using PostMan or RESTClient.

However, if you want to make this call in your code you will need an access token and make a similar call as above to get the information.

$tenantPrefix = "m365x162783";
$clientId = "Client-ID";
$clientSecret = "Client-Secret";
$tenantName = $tenantPrefix +".onmicrosoft.com";
$tenantDomain = $tenantPrefix +".sharepoint.com";

#Provide site url
$sitePath = "https://m365x162783.sharepoint.com/sites/Test1"
$siteName = $sitePath.Split("/")[4]

# use splatting
$ReqTokenBody = @{
Grant_Type    = "client_credentials"
Scope         = "https://graph.microsoft.com/.default"
client_Id     = $clientID
Client_Secret = $clientSecret
} 
$TokenResponse = Invoke-RestMethod -Uri "https://login.microsoftonline.com/$TenantName/oauth2/v2.0/token" -Method POST -Body $ReqTokenBody

$apiUrl = 'https://graph.microsoft.com/v1.0/sites/'+ $tenantDomain +':/sites/'+ $siteName +'?$select=id,displayName'
try {
  $spoResult = Invoke-RestMethod -Headers @{Authorization = "Bearer $($Tokenresponse.access_token)"} -Uri  $apiUrl -Method Get 
  Write-Host "Site:" $spoResult.displayName
}
catch {
  Write-Output "Failed to enumerate the site"
  Write-Host "StatusCode:" $_.Exception.Response.StatusCode.value__ 
  Write-Host "StatusDescription:" $_.Exception.Response.StatusDescription 
  Exit
}    

# NOTE THE SITES-ID is passed for the next call.
$apiUrl = 'https://graph.microsoft.com/v1.0/sites/'+ $spoResult.id +'/permissions'