Hello @Joel Agustin ,
Thanks for reaching out.
Salesforce Sandbox supports SP and IDP initiated SSO when installed through the Azure AD gallery, whereas Salesforce only supports SP initiated SSO. Therefore, you must create custom enterprise application, as shown below for Salesforce, so that the Sign on URL field becomes optional. I hope this was helpful.
Salesforce sandbox: https://learn.microsoft.com/en-us/azure/active-directory/saas-apps/salesforce-sandbox-tutorial#scenario-description
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