MS CRM 2015 On-premise - SSRS Report Connection Issue

I am writing a SSRS report for CRM 2015 on premise, and trying to create a connection to database.

I could able to connect to CRM SQL server through windows authentication from developer VM.

But I need to connect through SQL Server authentication using CRMadmin credentials(this user is used for installing CRM, SQL,also Domain Admin) , which is failing now, says "Login failed for user "DOMAIN\CRMadmin"

What should i do?
Should i create another SQL authenticated DB USer? what all permission should be granted to so that i can use that credentials in SSRS reports going forward?!!!!

How do i resolve this issue of connecting to SQL in SSRS reports which shouldnt cause any access problem when i upload .rdl to CRM  of any organization in a server.?

Please help