How am I suppose to get the txid of this transaction?
The same way you get the txid of any other transaction, by hashing it (unless it is segwit). Note that the transaction you linked to is not a segwit transaction, so you can just take its hash.
If the transaction is a segwit transaction (as indicated by the marker and flag bytes of
0x0001 following the version bytes), then you need to only hash the parts that are non-segwit. Specifically, you drop all of the segwit parts (so the marker and flag and any witnesses which can be found between the last output and the locktime), and hash what is left. This is described in BIP 144.
And how is it different from a regular (without multisig) one?