Doc: https://help.jiraalign.com/hc/en-us/articles/115000374374-10X-SSO-Settings

Currently states:

*Sign In URL. *This option is only available when the manual sign in is disabled. Set this value to a URL that you want to direct users to when they try to access Jira Align and are not signed in. Use the token %AC-URL% in the URL to include the Jira Align requested URL in the redirect.

As per bug JIRAALIGN-3901 which was closed with the statement

Our current SSO solution supports redirecting to a specific URL only if your Identity Provider is set up to support redirecting a user to a specific URL (versus sending something like a relay state and expecting Jira Align to perform the redirect).

To better support this, we'd need to support service provider-initiated SSO (versus identity provider-initiated SSO). We're going to set this ticket to closed for now, but would encourage you to follow the associated ideas in the Idea portal.

Please could we provide explicit information into the 10X SSO Settings documentation similar to:
We do not support URL redirect for Service Provider-initiated SSO such as Azure Active Directory, Okta, Idaptive

so that customers are clear on the limitations of this.