Hello,
The easiest approach would be to use a calculated field here.
{project}.{DE:<customFieldName>}
You are also able to get this reference into an external lookup field. Here you will have the disadvantage, that the external lookup field is a dropdown select field which always requires an initial choice.
Base API URL
https://yourinstance.workfront.adobe.com/attask/api/v19.0/TASK/search?ID={ID}&fields=project:DE:<customFieldName>
JSON Path
$.data[*].project["DE:<customFieldName>"]
Regards
Lars