Hey this may be a stupid question but how do you control a number in a message box? It only has 1 inlet

I'm using message 'set 1' to control object 'block~'. (Controlling the blocksize for a delay)

I want to use a controllable knob to change the numerical value. I thought 'set $f1" would take the float that would come in from the knob.

Appreciate any help

Cheers

cheers