> In the settings, I go to 'redirect to a custom finish page' and upload a htm file. After this,
> it shifts over to 'redirect to external website' and has the .htm extension in the web address.
This is the normal and expected behavior. The two options -- (1) custom finish page uploaded to the millisecond.com server and (2) redirect to some external URL -- are functionally identical, i.e. they boil down to the same thing.
> When following this URL, it takes participants to a blank page.
To see why that is, I'd need you to please post the link to your experiment's start page here.