Access BPF(Business Process Fields) Fields and Header Fields from JavaScript - Moazam Badane's Blog

The traditional way of accessing BPF fields is to append "header_pprocess" to schema name and "header" to fields on the header of the form. 

There is high probability of this command getting deprecated in the near future.

Dynamics CRMv9.0 made a client API available for developers through which we can get control's of whole forms as a collection.

If you want to lock "hsl_Amount", you use below code this would return controls as a collection and iterate through each loop to make control disable. 

formContext.getAttribute("hsl_Amount").controls.forEach(function(control, index) { control.setDisabled(true); });

I would not recommend hard coding this in JS because it might break in a future release of CRM.