How to create the data parameter for a PSBT?
A PSBT contains an unsigned transaction that has all of the inputs and output that you want. The other data PSBT allows for are put in around that unsigned transaction.

Here, data just means that the unsigned transaction will have an output that has an OP_RETURN followed by whatever data you want. This directly reflects what createrawtransaction. data is not specific to PSBT nor is it necessary.