You are creating a new HTML mashup of type Code, however is not able to find the custom fields to be used as port binding.
SAP Cloud for Customer
Reproducing the Issue
1. Go to Administration workcenter.
2. Open Mashup Authoring.
3. Search for mashup "ABC".
4. Check on Input Parameter.
5. No custom field is available.
Custom field are never part of any SAP delivered port binding.
If you want to pass data of any Key user created field, then you should keep parameter as empty and while adding the mashup to screen during the adaptation, you can map the parameter (With blank value) to custom field. This is applicable to any HTML mashup of type URL only and not Code.
Other way can be to create custom port binding with custom fields.
Mashup; Fields; Missing; Custom; KUT; Extension , KBA , AP-RC-GEN-MSH , Mashups , How To