Hi jitendrat76652144
There is no built-in script for SMS External Account Setup, there are built-in scripts for Push Notifications. You could check for the same under Administration -> Configuration -> Javascript Codes. These are by the name of iOSUtils.js and androidPushConnector.js.
In case you wish to add your own customized Javascript Code for setting up an SMS Account with third party, feel free to add it within the same section by clicking on the New Icon and then referencing this Script within the Connector tab of the External Account.
However, it is strongly recommended that any such customized Scripts be implemented and added with the intervention of Adobe Consulting. These are outside the scope of Adobe Campaign Support and in case of any issues, they might not be able to assist you without Consulting's insights.