In my test I can use a variable as default value:
- store | 123 | var
- prompt | hello@${var} | var2
Does this work for you? Maybe you forgot to define your variable? 
{
"Command": "store",
"Target": "123",
"Value": "ddd"
},
{
"Command": "prompt",
"Target": "hello abc@${ddd}",
"Value": "kkk"
},