If you have a standard UserVoice domain, e.g.
https://<subdomain>.uservoice.com, you can setup SAML SSO with OKTA by adding UserVoice from the selection of OKTA apps. Follow this guide.
If you have a custom domain (CNAME), e.g.
https://ideas.myapp.com, you will need to setup a custom SAML App in OKTA. Please follow this guide.