Require a Managed Object
In some environments, or for some ticket types, there always needs to be an Asset. After all it doesn't make much sense to have an install software request without a computer seleted. For those options you can create a validation rule that will require this option.
If creating your validation rule for something such as as "Request" Type just enter it with the following parameters:
Name: Asset must be selected
Comment: Requires workers to enter select an Asset when none has been selected.
Display this message: Please select a valid Asset. The Asset NONE is not valid for requests.
When: Every time incident is saved
When ALL of these are TRUE
"Type" is not equal to "Request"
"managed_object_id" is the same as "1"
To enter the last line of that parameter just use an <Advanced Condition>