BEanshell Preprocessor?

Hi,
we have code in a beanshell preprocessor that is run for each sampler (100)+ for each thread.
It only needs to be processed once for each thread just before the first element in the thread.
My structure is:

test plan
beanshell preprocessor
thread group
transaction controller
logic controller-include
sampler2...
sampler110

Where should the beanshell preprocessor be placed so that it is run only once per thread (user)?
Is it a beanshell preprocessor that's needed or a different element?

Thank you.