How to populate the customer name in EFF using Pretransformation rules in Sales order form

Summary

we got a requirement to populate a EFF value based on the customer selected, as we dont have feasibility of adding customer name in the value set of EFF as a workaround we are trying to implement it by creating an extra EFF field(custname) to populate customer name, populating the custname using pretarnsformation rules and then using the custname EFF value in the orginal EFF value set. To achieve populating custname EFF with customer name i am trying to get the customer name in the pretransformation rule which is not working.

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