I have a memo field, and I am trying to extract a user name that is stored is the memo. It has text right before the user name stating 'Placed_by:'

If you want to ignore whatever is in front of or after the user name, try

Like "*" & [Enter User Name] & "*"

OR

If you want to search only the memo fields that have "Placed_By: " use

Like "Placed_By: " & [Enter User Name] & "*"