Should be as simple as copy pasting server url + credentials or streaming key. My interpretation is that documentation authors either considered it so simple that it's not worth clarifying or since they are pushing for their own software (rumble studio) they intentionally didn't explain it further to encourage people using their software directly without extra step of OBS.
Once you open corresponding panel in Rumble it shows you all the information you need to copy paste in single place with convenient copy to clipboard buttons.
Platforms with "builtin support" require same amount of configuration minus the server url. But since the server url is provided in the same place as stream key, copy pasting 2 instead of 1 value hardly makes a difference. It probably takes less time to copy server url than trying to find it in dropdown containing 70+ other services.
Once you open corresponding panel in Rumble it shows you all the information you need to copy paste in single place with convenient copy to clipboard buttons.
Platforms with "builtin support" require same amount of configuration minus the server url. But since the server url is provided in the same place as stream key, copy pasting 2 instead of 1 value hardly makes a difference. It probably takes less time to copy server url than trying to find it in dropdown containing 70+ other services.