Update field according to other field's value

Frappe Framework Custom Script
2

Hello;
This is possible using custom code as following:

frappe.ui.form.on('Timesheet Details', {
        activity_type: function(frm, cdt, cdn) {
                var item = locals[cdt][cdn];
                if (item['activity_type'] == 'Billable') {
                        item['bill'] = 1;
                }
        },
});

This can be added as custom script or can be added in separated custom application but in this case there are steps that you need to proceed in it, please check this link as reference for you:

Regards
Bilal