Need help with creating a trigger after insert

I've the following tables:
table1 - LotTransactions: StockCode, BinNumber, LOT, Date , PO, Source
Once new record is added I need to update lot number which would equal existing lot number + 4 random digits.
This update should only happen if Source is P.
table2 - LOTDetails: StockCode, BinNumber, LOT
Table2 LOT needs to be also updated by adding the same 4 digits. as in table1.

For example:
Transaction table has this records after insert
product10 , bin02, lot100, currentdate, PO567, P
LOTDetails table
product10, lot100, qty, bin02

After Trigger it should look like below:
Transaction Table - product10 , bin02, lot100.6548, currentdate, PO567, P
Lot Detail Table - product10, lot100.6548, qty, bin02

Your help is greatly appreciated.

Thanks You

Don't do this.
But if you must something like this
You need to consider the unique indexes on tjhe tables and bear in mind that a random number isn't unique.
This is a really bad idea.

drop table [transaction]
drop table LOTDetails
create table [transaction] (StockCode varchar(100), LOT varchar(100), Source varchar(1))
create table LOTDetails (StockCode varchar(100), LOT varchar(100))

create trigger tr_transaction on [transaction] for insert
declare @a table (StockCode varchar(20), Lot varchar(100))
insert @a select StockCode, LOT+'.'+right(convert(varchar(100),checksum(newid())),4)
from inserted
where Source = 'P'

update [transaction]
set Lot = t.Lot
from [transaction] f
	join @a t
		on f.StockCode = t.StockCode

update LOTDetails
set Lot = t.Lot
from LOTDetails f
	join @a t
		on f.StockCode = t.StockCode

insert LOTDetails select 'a', ''
insert LOTDetails select 'b', ''
insert LOTDetails select 'c', ''
select * from [transaction]
select * from LOTDetails
insert [transaction] select 'a', 'a', 'P' union all select 'b', 'b', 'P' union all select 'c','c','X'
select * from [transaction]
select * from LOTDetails