Set Site field according with email address

Set Site field according with email address

Hi, 
I try to automate to set the site field according to email suffix address and was wondering if get some advice how to do it. 

E.g. receive request the form  mlo@dukesacademy.org.uk  the site field will be filling it up with Dukes, if the requester uses the address  mlo@oakspark.redbridge.sch.uk  it filled it up with Oaks Park. 

I already created this sites and tried to use forms rules to achieve it but is not working. 

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