Need guidance with redirect page that submits html form data

I'm a SP Admin but not a full developer so be gentle with me as I may not follow deep coding comments.  I'm migrating from SharePoint 2016 On Premise to SPO with Modern pages.  I currently have a page on-prem that when the user opens the page it redirects the users browser to a separate system using a POST to submit a HTML Form containing variables for the external site.  That page errors out when copied to SPO so I'm looking for guidance on ways to continue this process and have an easy process to modify the credentials down the road as they change every 90 days.