https://hi.service-now.com/kb_view.do?sysparm_article=KB0749543

This knowledge article helped:

I added following code on line 59 and defined mid server in Discovery Credentials :

// Now we have everything set up - call the mid selector API to select one mid
var ServiceAccount = JSON.parse(context).service_account_id;
//gs.log("MK-Checking ServiceAccount="+ServiceAccount);
var credential='';
var gr = new GlideRecord("cmdb_ci_cloud_service_account");
gr.addQuery("account_id",ServiceAccount);
gr.query();
if(gr.next())
{
credential=gr.discovery_credentials.mid_list;
// gs.log("MK Credential is = "+credential);
}
if(credential!='')
{
return credential;
}
else
{
return midSelector.selectMid(app, null, capabilities, context);
}
},