You have created a custom KUT Workcenter and Workcenter View and are trying to add a mashup on it. However, you are not seeing the mashup in the list to be added in the screen.
SAP Cloud for Customer
KUT custom Workcenter and Workcenter View not support URL Mashup. It support only HTML Mashup.
Instead of creating a URL mashup you should create a HTML mashup.
Mashup; KUT workcenter , KBA , custom , mashup , AP-RC-GEN-MSH , Mashups , How To