You are creating a HTML mashup type Code and is not able to find the KUT field as parameter.
SAP Cloud for Customer
Reproducing the Issue
1. Go to administration workcenter.
2. Open the Mashup Authoring.
3. Click in New then HTML mashup.
4. Set the name and port binding.
5. Select HTML Code type.
6. You are not able to find the KUT field under Input parameter.
This is applicable to any HTML mashup of type URL only and not code.
Custom fields are never part of the 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 adaptation, you can map the parameter (with blank value) to custom field.
Other way can be to create custom port binding with custom fields.
KUT, Field, Mashup, Parameter, Missing , KBA , AP-RC-GEN-MSH , Mashups , How To