You can use your OneLogin credentials to sign in to HackerOne. To set up SSO via SAML for OneLogin:
Log in to OneLogin.
Click on Administration.
Go to Apps > Add Apps.
Search for the HackerOne application and select it.
Click Save on the Configuration page.
Copy the X.509 Certificate and the SAML 2.0 Endpoint (HTTP) on the SSO tab. Click View Details to get the full X.509 Certificate.
Open HackerOne in a new tab.
Follow the SAML setup instructions here.
Paste the Single Sign-On URL and the X.509 Certificate that you copied from Step 6 into the corresponding fields during SAML setup.