Witness Plutus script hash --> how to calculate?

Hi All,

Having a little trouble figuring out how to calculate the Witness (hash) of a plutus script?

For instance i initiate a SPEND of an input at a script address (plutus contract), at an address:


and recorded on the chain i have a:

witness (plutus script: c70fa618ed1efc418b215408a54f9fbdfb2386954e93bf2967ead4ba3945a55f ) within the relevant transaction data (datum hash, redeemer etc)

how is this value calculated?

Could anyone point me in the right direction here. thankyou