Fetching recent Salary Structure Assignment

I have inserted custom field for Salary Structure Assignment in Additional Salary doctype,
The field name and field label are ssa and Salary Structure Assignment respectively,
How do I fetch the recent Salary Structure Assignment of employee.

From Anurag, following code works.

setup(frm) {
    // This will fetch all SSA options if in case you want to change. 
	frm.set_query('ssa', function(doc) {
		return {
			filters: {
				"employee": cur_frm.doc.employee
	    //this will fetch and set the most recent assignment.
    frappe.db.get_link_options('Salary Structure Assignment', '',
			employee: cur_frm.doc.employee
	).then(all_ssa =>{
	    frm.set_value('ssa', all_ssa[0].value)