How to check the mandatory field that contains default value, is added or not through Client Script?

[Rutuja1]

I have a field in a table which is mandatory field.

Now I have provided a default value for that field. But I want User to fill that field.

it should be like default value will be there and user should suppose to fill that mandatory. Or else the form should not submit.

How do I do this via client script 'onSubmit'? 

[SD13]

I have a similar requirement but it is for a variable on a Catalog Item in the Service Portal so this approach won't work. Any ideas?