You want Bitrise to connect to a private Git repository or submodule using HTTPS authentication instead of SSH.
You can use the Authenticate host with netrc Step, which can store your Git authentication info. Simply use this Step instead of the Activate SSH Key Step. The two Steps are compatible with each other: you can use both in the same Workflow for different repositories.
Please note that if you wish to access the main repository of your app with HTTPS authentication instead of SSH, you also need to change the repository URL. To do so, open the app's page on Bitrise, go to the Settings tab, and find the REPOSITORY URL section.