I think the functionality has already been mentioned in the doc, give a sample here to elaborate on the specific usage.
For example, you want to assign the Application Administrator role to a user, then the script should be:
Note: The -ResourceId parameter uses your AAD tenant id <tenant-id>.
$role = Get-AzureADMSPrivilegedRoleDefinition -ProviderId aadRoles -ResourceId "<tenant-id>" | Where-Object {$_.DisplayName -eq 'Application Administrator'}
$schedule = New-Object Microsoft.Open.MSGraph.Model.AzureADMSPrivilegedSchedule
$schedule.Type = "Once"
$schedule.StartDateTime = (Get-Date).ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffZ")
$schedule.endDateTime = "2021-07-25T20:49:11.770Z"
Open-AzureADMSPrivilegedRoleAssignmentRequest -ProviderId aadRoles -Schedule $schedule -ResourceId "<tenant-id>" -RoleDefinitionId $role.Id -SubjectId "<object-id of user or group>" -AssignmentState "Eligible" -Type "AdminAdd"

Check in the portal:
