Yeah!
So, if you had a JSON like:
{
"k1": $v1,
"k2": "$v2"
}
Then we can’t really replace $v2 with the variable value, as then there would be no way of distinguishing a simple json string and a variable.
We only replace $v1. Variables are supposed to be present without quotes, otherwise we can’t distinguish a string from a variable. Someone may actually be wanting to just send “$v2” as the value.
So, you will need to write a separate backend app to do the part of forming the graphql± query, and use that with @custom.