Format for 'openid connect authorize parameters' field

We are happy users of the openid-connect plugin. Setup was very straightforward.
We do need an additional parameter in the authorization url.
Specifically for us: kc_idp_hint=hbso
I have tried all specification variations I could think of in the settings. But nothing shows up in the url.
Probably dooing something wrong?
I now solved it with a hack in the 'def authorize_params’function in omniauth_openid_connect.rb
where I added session[‘omniauth.kc_idp_hint’] = params[:kc_idp_hint] = ‘hbso’
But it must be possible to use the appropriate setting field, I think.