Lists in Scheme are linear linked structures, which makes it necessary to apply linear search techniques.
Strings are also linear structures, but based on arrays instead of lists. Thus, strings can be linearly searched, but it is also possible to access strings randomly, and more efficiently.
First, design a function which searches a string linearly, in the same way as find-in-list. Will you just replicate the parameters from find-in-list, or will you prefer something different?
Next program your function in Scheme.