Git credential helpers work via writing and reading from standard input and standard output which helped me debug an issue I was having with pass-git-helper. To get a credential helper to spit out the password, invoke it with the get action and pass in some basic key=value data. The interaction looks somewhat like given below, with < indicating the standard input of the credential helper and > indicating its standard output.
# for URL https://git.msfjarvis.dev/msfjarvis/pass-store
$ pass-git-helper get
< protocol=https
< host=git.msfjarvis.dev
< path=msfjarvis/pass-store
> protocol=https
> host=git.msfjarvis.dev
> username=msfjarvis
> password=hunter2