Auto populate manager field from a lookup select box field

Hi all,

I have two fields namely User and Manager on a catalog form. Both these fields are of lookup select box type. The look up table for both is sys_user table. 

I want to populate the value of manager field as soon as someone fills user field i.e The user which is selected in the user field, his/her manager name should populate on the manager field.

I have seen a lot of articles regarding reference field but there is nothing related to look select box field. Any help would be appreciated,

Thanks