Fixed!!!
I have managed to implement it in another way. As I the required="true" didn't work as I wanted, I wrote the code in WorkOrderEditController.js file.
To make the Description field required in my Create Follow-up WO page,:
1) I added the following code in the validateFields() function in WorkOrderEditController.js file.
if ((this.uiRequired("description", workorder.description)))
{
arrayListFieldsWithError.push({"attributename":"description", "error":true});
} else {
arrayListFieldsWithError.push({"attributename":"description", "error":false});
}
2) Added the below 2 properties in my smart-input XML tag for Description:
on-blur="validateFields" required = "true"
With this, Save button gets disabled when Description value is empty.
Code can be written in AppCustomizations.js as well, but need to see how does that work.
Thanks.