Authenticating against the Azure REST API can be very tricky. Usually you are advised to create an App Registration/Service Principal, get the Cliend Id and create a Client Secret, use those to get a token and authenticate with said token against the request.
There is a much simpler way - using az rest.
To authenticate and invoke a custom HTTP endpoint, you have to have az cli installed and be logged in. Using az rest automatically authenticates using the logged-in credential: If Authorization header is not set, it attaches header Authorization: Bearer <token>:
az rest method get --url "https://management.azure.com/providers/Microsoft.Billing/billingAccounts?api-version=2019-10-01-preview"
Or with PUT method:
az rest --method put --url "https://management.azure.com/providers/Microsoft.Billing/billingAccounts/4858737/billingRoleAssignments/326c745a-d97d-46d0-9d81-8b49545d09c3?api-version=2019-10-01-preview" --body @body.json