Hi Shafeer,
To clarify the situation, I'd like to confirm whether you want to purchase or trial a new Office 365 subscription in your original tenant account or create a new Office 365 tenant account (e.g. *** Email address is removed for privacy ***).
If you want to create a new Office 365 tenant account, you may purchase or trial an Office 365 for Business subscription via
https://products.office.com/en-us/business/compare-more-office-365-for-business-plans, and some of the subscriptions (e.g. Office 365 Business Essentials, Office 365 Enterprise E1, etc.) don't have the free trial version.
If you want to purchase or trial a new subscription in the original Office 365 tenant account, you may log into
the portal >Click Subscriptions under
BILLING> Add subscriptions.
For "Add client domain to it", I'd like to confirm whether you want to add an domain used in the previous Office 365 tenant account to a new Office 365 tenant account.
If so, I need to explain that one domain can only be added to one Office 365 tenant account. If the domain has already been used in a tenant account and you want to add it to a new tenant, you need to remove the domain from the
previous tenant account and then add it to the new one.
If anything is misunderstood, please let us know.
Thanks,
Felix