My country ( i.e. Bangladesh) is not listed on the billing information? My country is not listed in the billing system. I traveled from Saudi Arab to Bangladesh and i wanted to add the billing information with my Bangladeshi Number.  It seems that the Payment method doesn’t have my country listed.How do i add my country ( i.e. Bangladesh)?

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Apple lists the countries where services are available when you choose your location.

https://www.apple.com/choose-country-region/
https://www.apple.com/ios/feature-availability/
https://www.apple.com/macos/feature-availability/
https://www.apple.com/watchos/feature-availability/
etc...

As of January 2019, the only Apple Features offered in Bangladesh is Maps and Satellite Maps. Also, keep in mind that the features are enabled in almost all cases based on the region where you purchase the device, so if your phone was purchased in Saudi Arabia and then used in the US - you might retain all the Saudi restrictions despite your attempt to change. App Store and other services for billing purposes typically are more flexible and you can take advantage of them when you move about.

My speculation and research why Bangladesh isn’t listed is due to Apple not obtaining some rights from the government or is in the process of obtaining them.

In order to run an iTunes store in any given country, Apple has to secure licensing rights for the content, legal rights from the local government, payment processes, and numerous other legal hurdles. Either Apple hasn't applied for those rights yet, or they're in progress, but the size of the user base as well as the ease (or otherwise) of doing business in any given country are the driving factors here.
You can add your voice via Apple's feedback page here: http://www.apple.com/feedback/itunesapp.html but at the end of the day you can be the most ardent, vocal fan in the world, but if the Bangladesh government don't give the licenses to operate, it all means nothing.

(Source)