How to populate parent account lookup upon lead conversion?

Hello,

You need to create trigger like below. Change parent account API name

trigger populateParentAccount on Account (before Insert){
    List<Lead> convertedLeads=[SELECT Id, ConvertedAccountID, Lead_Parent__c FROM Lead WHERE IsConverted=True AND ConvertedAccountId IN :trigger.new];
    Map<ID,ID> acctParentMap=new Map<ID,ID>(); 
    for (lead l: convertedleads){
        acctParentMap.put(l.ConvertedAccountId,l.Lead_Parent__c);
    } 
    for (account a:trigger.new){
     if (acctParentMap.containsKey(a.Id)){ 
     a.ParentId = acctParentMap.get(a.Id); 
     }
    }
}