[exim] How to set up command with arguments in ${run ...} ?

How exactly do I execute a command with arguments via ${run ...}? Based
on the Exim spec, I thought I might do something like

${run{/bin/echo tst}{$value}}

(and get the output of "/bin/echo tst", i.e. just "tst"), but if I try
this, the exim log says:

failed to expand "${run{/bin/echo": missing } at end of string

Exim version is 4.30.

- Toralf