I haven’t solved this, and I doubt I ever will.
The languages Brainfuck, Intercal and Whitespace allowed for the SBSTR1 problem are intentionally designed to be awkward. Looking at the submissions, I see that you’ve chosen BF.
I found this page which might help, and I also found a BF repository on Github that might help with some code examples.
Perhaps one day someone who has solve this problem will provide a better answer with more concrete help.