Fields (Custom)
Custom onboarding fields are fully supported inside Dynamics 365.
First, review your onboarding as submitted by the vendor:
You can also view details of all fields available in the current onboarding form by section.
This section will show all sections within onboarding form.
If there's a custom field, it will be shown withing Custom section(s).
For default onboarding field, there will be standard Field ID starts with alphabetic letter which links to existing D365 field.
For custom field, Field ID will be numeric and won't show in Vendor master form.
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Customisation and extensions can be written using code extensions