Hi
you have to write loadcontent event js to prepopulate values.
In JS you have to add a item first based on the condition check and add values to that multifield.
example:
loadcontent js
if(multifield.getValue()=='')
{
multifield.addItem();
if{
here you can write your logic to set prepopulate values.
}
if you need any further help. just let me know.
Thanks.