Getting a group from API Rest

Hi, Im using Bonita 7.3.3

Im using the structure provided by Bonita (organitation, groups, roles...). In a form, I load a selector with the groups information with the call API:

../API/identity/group?p=0&c=100

When the user select one item, the value (id) is saved in a variable and later in a field for a BDM called "X".

In another form, I have a container with a selector. The selector load the records of "X" bdm. I want that when user select a record, in a input show the "displayName" of the "group" saved at "X". I have done this with two vars:

var1 = "../API/identity/group/{{var2}}"
var2 -> value from the input.

This work fine, but the problem is that the container have a collection, so the selector value is saved at $item.var, and I dont know how inject this value at the api call for recover dinamically the displayName.

Thanks