How to set my custom class in a business process written manually, not BPL (contextsuperclass)

I need to set and use some context variables in a transformation (written in DTL), but the code is manually written.

When writing the same as BPL I define context variables and they are visible in transformations called by that process.

In something like this my.DTLtransform sets HL7 message fields to values in context variables:

Set dtl = ##class(my.DTLtransform).%New()
Set qStatus = dtl.Transform(pRequest, .msg)
 

However, when I create a business process using custom code option, I don't see an obvious option, as well as documentation covering that area.