Try below JavaScript code to set multiple values to lookup control.

NWF$(document).ready(function()
{
   NWF$('#' + ddlmulti).val('5;#;#6;#;#7');           //append all lookup ids that you want
});

This works fine in document ready function , but the same is not working on change event

NWF$('#'+ddlmulti2).change(function(){
NWF$('#' + ddlmulti).val('5;#;#6;#;#7');
});

Is there anything to be changed on the above script?

I tried this in Ninex Forms for Office 365. But it's not working.
Which version of Nintex Forms are you using ?