Is there a verb for replacing placeholder values with actual values in a template?
The verb for transforming "Hello FirstName LastName" to "Hello John Smith".
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One possibility is you populate the template (that's over 1000 written instances in Google Books).
Note that this use of populate is a bit "geeky", and thus likely to be restricted to software contexts. For more general contexts, it's worth noting that Google Books has nearly 9 times more instances of fill in the template.
The verb is
meaning to represent something by an instance. The act of instantiating is an instantiation:
the representation of (an abstraction) by a concrete example (logic)
the process of deriving an individual statement from a general one by replacing the variable with a name or other referring expression
I think you can use substituted. I derive this from @deadrat's comments above - "The OP's situation seems to call for 'parameter substitution.'"
use or add in place of/replace
In your case,
The placeholders, firstName and lastName were substituted with the actual values "John" and "Smith" respectively.