I need to filter on a lookup field when using CAMl Query.

I have the below function. I need to modify the caml to filter on a lookup field. I have not had much success. Help is appreciated.

function getAdditionalApprovals()
{
     var ListName = 'Additional Approvals';
            var ctx = SP.ClientContext.get_current();
            var list = ctx.get_web().get_lists().getByTitle(ListName);                              
            var caml = SP.CamlQuery.createAllItemsQuery();          
            caml.set_viewXml("View><Query></Query></View>");
            this.allitems = list.getItems(caml);
            ctx.load(this.allitems);
            
            ctx.executeQueryAsync(Function.createDelegate(this, this.onAdditionalSSucceededCallback ), Function.createDelegate(this, this.onAdditionalsFailedCallback));

}

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faye fouladi