Open rent do have a ‘room only’ agreement option. I use this for HMO. Unsure if that’s suitable for lodger, as I’m not knowledgeable in that market.
If you want to use your own agreement, this won’t be via Openrent. Having used them for the tenant find & checks, now you need to make your own arrangements for tenancy signing, as you would normally.
I doubt if a room only ‘tenancy’ agreement is suitable for a lodger, but you can adapt one for that purpose. A lodger agreement needs to incorporate your house rules, as you must live there and share your facilities for them to be classed as a lodger.