Hi Mita,

Please find the solution. "When new opportunity is created, I would like to auto populate contact role."
we can write trigger for this i.e when the opportunity is created then opportunityContactRole is also created.

trigger OpportunityTrigger on Opportunity (after insert) {
    
    if(trigger.isAfter && trigger.isInsert){
        set<Id> accountSet=new set<Id>();
        for(Opportunity op:trigger.new){
            accountSet.add(op.accountid);
        }
        List<Contact> contactList=new List<Contact>([select id,accountid from Contact where accountid in: accountSet]);
        List<OpportunityContactRole> ocrList=new List<OpportunityContactRole>();
        for(Opportunity op:trigger.new){
            for(Contact con:contactList){
                 if(op.accountid==con.accountid){
               OpportunityContactRole ocr=new OpportunityContactRole();
                ocr.ContactId=con.id;
                     ocr.OpportunityId=op.id;
                      ocrList.add(ocr);
            }
            }
        }
        if(ocrList.size()>0){
            insert ocrList;
        }
    }
}

Please let me know it is working or not?

if it helps you please mark it as the Best Answer so that other people would take reference from it.
Thank you!