There may be cases where you need to set one lookup field value into another lookup field on the form using JavaScript in Dynamics 365 .

What is Lookup field?
A Lookup field type represents the relationship attribute on the related entity.

Required Attributes of lookup fields:

id: The GUID of the item. Required for set.
name: The name of the item to be displayed. Required for set.
entityType: The entity name of the item. Required for set.

SCENARIO:
On the Case form, we have two fields one is Owner (ownerid) of the Case and another one is Custom field called Developer of the Case (adyt_developerofthecase). We need to set Owner Lookup value to the Developer of the Case Lookup on the Case form using JavaScript.

JavaScript Code:
Getting Owner Lookup of the Case.

function getLookupDetails(executionContext)
{
// Getting Form Context
var formContext = executionContext.getFormContext();
// Getting the value of Case Owner id
var OwnerId = formContext.getAttribute(“ownerid”).getValue();
// Getting the GUID of the lookup record
var Id = OwnerId[0].id;
// Getting Name of the lookup record
var Name = OwnerId[0].name;
// Getting Entity Name of the lookup which entity, the lookup record is belonging to.
// Getting Entity Name of the lookup which entity, the lookup record is belonging to.
var EntityType = OwnerId[0].entityType;

Setting Owner lookup of the Case to Developer of the Case
var lookup = [];   // Creating a new lookup Array
lookup[0] = {};    // new Object
lookup[0].id = Id;  // GUID of the lookup id
lookup[0].name = Name; // Name of the lookup
lookup[0].entityType = EntityType; // Entity Type of the lookup entity
// Getting Developer of the Case and Setting Owner of the case to Developer of the Case
var OwnerId = formContext.getAttribute(“adyt_developerofthecase “).setValue(lookup);
}

Output:
Developer of the Case should be the Owner of the Case.