if I need the SaS token will be the one which I used for registering
device using Device Provisioning Service
There are some steps that you need to follow to generate the access_token that will be used in the Authorization header of the request.
Prerequisites:
Below are the steps:
Login with Azure CLI
az login
Set Active Subscription
az account set --subscription "your subscription name or id"
Create Service Principal
az ad sp create-for-rbac -n "your service principal name. Can be like 'jongpostman7'"
The output of above command will be something like below image.

Copy this output to a temp location, you will need the values in a minute.
Coming to the Postman request now. You need to first call the AAD Token request whose response will provide you with the access_token. This access token you can use in the DPS delete device request. The AAD request will be like below:-
POST https://login.microsoftonline.com/{{tenantId}}/oauth2/token
Refer image below for the request body.

grant_type is client_credentials
client_id is the appId received in above command output
client_secret is the password received in above command output
tenant_id is the tenant received in above command output
resource is https://management.azure.com/
The response of above request will be like below:-

Now, you have the access_token that can be used in the DPS delete device REST API.
The Authorization header be like below:
Authorization: Bearer <access_token_received_above>
Reference link here.