So ... first let me say that, pretty obviously, stakeunited is an independent entity - no affiliation with B3 coin at all.  You invest there at your own risk, and probably should do research on them before you do.  I know a lot of people stake there ... I don't, so can't say anything about the site or staking there.

That said, your question seems to also ask about the tx id ... which is the transaction id and where you can find it.  After you do your withdrawal from coinexchange, you can find the tx id by clicking on the "My Account" button in the upper right of the screen, scrolling down the list to "Withdrawals" ... and in that screen you will see your recent withdrawal (most likely the top on the list) and there is a column that says "Receiving Address / txid" on the right of the withdrawal info ... the tx id is the long number under the address you sent your coins to.  Copy that into the Stakeunited form.

Good Luck.