Error when using $ in G code

In the "Tool On Sequence" box if I enter `M3 $1` the following error is emitted and the save button is disabled:

```
error: could not parse gcode
  ┌─ <input>:1:4
  │
1 │ M3 $1
  │    ^ expected one of EOF, checksum asterisk, letter, letters, newline, opening parenthesis, semicolon, or whitespace
```

This is how to select a specific spindle in linuxcnc.