If the case in question does not currently have an OYPID number, you can add one manually via the menu titled Transfers, where, upon using the New Transfer button, you can populate the OYPID field, which will, in turn, populate the value seen on the case details and will cascade through to AssetPlus.
If you aren't sure what to set as the Transfer Date, this can just relate to when the service first started working with the young person. As the other fields are not mandatory, it should then just be a case of adding the OYPID value.
With regards to locating the correct OYPID for a young person, information can be found within the YJB Data recording requirements guidance, which is published online. You can find this on the YJ resource hub site.
The format is essentially the local system ref, for example, the YP ref, and two characters from the YOT OU Code. You can check this in the LU_YOT table for your LA OU code, for example, for Wandsworth, this is F00WW00, so you’d use WW and the case start date in the format YYYY/MM/DD.
So you could have 1234WW20221221.