If it is in fact Bitwarden that is suggesting the Web Vault credentials, then all you have to do to fix this issue is to edit the entry of vault.bitwarden.com in the browser extension, find the “URI 1” field (which would have a value similar to https://vault.bitwarden.com/#/login), click the 
icon next to the URI field (which will toggle the visibility of a dropdown menu), change the dropdown menu selection from “Default Match Detection” to “Host”, and finally, click Save in the upper right corner.
As far as I know, Bitwarden does not have such a prompt. Are you sure that you are not seeing prompts from your browser’s password manager?
Was there ever an entry in the Bitwarden vault before you did this? It is possible that everything you have described so far is caused by the browser’s password manager, without any involvement of Bitwarden.
This is part of your problem. If you are going to use Bitwarden’s browser extension, you need to disable the Chrome/Google password manager.
If you have enabled “Ask to add login” and “Ask to update existing login” under Settings > Notifications in the Bitwarden browser extension (and if the website is not on your list of Excluded Domains), then you should get a prompt to save the login credentials in your Bitwarden vault. However, it is a known fact that this function does not always work on every website (whether the Community Forum website is one of the problematic sites or not I cannot confirm at this time). Therefore, I usually recommend that you disable the two options “Ask to add login” and “Ask to update existing login”, and instead use the method that @Neuron5569 linked to in the response above, which I reproduce here for your convenience:
Adding a New Login:
The procedure below assumes that you have the website’s account registration form open in your browser, and that your Bitwarden browser extension is currently unlocked:
1.Open the browser extension (click Bitwarden 
icon at the top of the browser, or press Ctrl+Shift+Y).
2. Click 
(or the “Add a Login” link).
3. Type the desired username (or generate a random one).
4. Click the 
icon the in Password field (generate password).
5. Click Select in the upper right corner.
6. Click Save in the upper right corner.
7. You will now see the new vault item listed at the top of the browser extension’s “Tab” page → click on the website name (which will transfer your username and password to the website’s account registration form.
8. In your browser, submit the account registration form to the website server.