try this i write for user_id field you can change whatever field you want to fetch.
frappe.ui.form.on("Sales Order", "sales_person",function(frm,cdt, cdn)
{
if(frm.doc.sales_person)
{
frappe.call({
"method": "frappe.client.get",
args: {
doctype: "Sales Person",
name: frm.doc.sales_person
},
callback: function (data) {
if(data.message.employee)
{
frappe.call({
"method": "frappe.client.get",
args: {
doctype: "Employee",
name:data.message.employee
},
callback: function (data) {
frm.set_value("p_email", data.message.user_id);
}
});
}
}
});
}
});