Azure AD B2C custom policy include acr_values in as a parameter

Hi, we use a 3rd party IdP that needs to get acr_values property in the authentication call, is there a way to add it?

Now we have to add it to the actual authorization url like this:

<Item Key="authorization_endpoint">https://idpservice.grean.id/oauth2/authorize?acr_values=urn:grn:authn:se:bankid:another-device</Item>

I cant get it to work by just adding another key like this:

<Item Key="acr_values">urn:grn:authn:se:bankid:another-device</Item>

Is there another way?

Thanks!