When a user selects a property (top level property, not a subproperty) of an instance in a service and selects the "TOSCA functions" button then a new "custom" option shall be added to the drop down list of TOSCA function types.

When the user selects "custom" they will then be able to enter a name for the custom function. They will also be able to add any number of arguments. The arguments can be values or value expressions (i.e. TOSCA functions), similar to the way arguments can be added to the concat function today.

Upon saving the value will be passed to the backend and handled by the supported added by SDC-4455