Command inside parameter

Hi colleagues !

Quick question:
How can I run a command inside a parameter ?

I want to use dd and the following formula in the count parameter :
df -m . |awk '{s+=$2} END {print s}'

like:

dd if=/dev/zero of=dummy.file bs=1m count= df -m . |awk '{s+=$2} END {print s}'

Thank you for all support !