Question: How to select field based on script variable

I have simple alert where field name may vary between tasks. How to write basically {{index .Fields hereIsMyVariableThatChanges}}

```
var myFieldName string
...
.alerta()
.value('{{index .Fields myFieldName}}')
...
```

Is there some syntax trick to do that? Or should i write some kind of .eval with .as?

Start with the expression examples in the issue, especially `.alerta()`, `.value(...)`, `.eval`, `.as`, and `.Fields`. Determine whether the requested variable-based field selection is supported and what the documented syntax or limitation should be; the issue provides no file or test to target.

Written by the indexing model from the issue text.