Custom ser_query filtered link field

i use set_query with link field.

frappe.ui.form.on("Issue Return Head", "onload", function(frm) { frm.set_query("srno", function() { return { filters : [ ["Stock", "mgrcode", "=", frm.doc.aapnar] ] }; }); });

i want to select only filtered srno.
problem:- but here i can write any srno.

in image i have filtered srno

but i also can write srno which not available in filtered. which in wrong

what i want :- user can select or write only filtered srno