In V2 QBO, Custom fields are supported for Customer and Job entities only. Ref - https://developer.intuit.com/docs/0025_quickbooksapi/0058_faq/qbo_v2_to_v3_migration_guide#Custom_Fields#Custom_Fields
You can try V3 QBO REST APIs - https://developer.intuit.com/docs/0025_quickbooksapi/0050_data_services/020_key_concepts/0200_custom_fields
V3 .Net Devkit (Custom Fields API Ref) - https://developer.intuit.com/docs/0025_quickbooksapi/0055_devkits/0150_ipp_.net_devkit_3.0/custom_field_apis
Thanks