This app will send the sms on the partners mobile number using third part service like msg91, href="https://msg91.com/ You have to create an account in order to send the sms, after creating account you will be able to see the auth_key, there are two type of messages Transactional and Promotional you can read more here https://help.msg91.com/article/64-what-is-the-difference-between-transactional-promotional-and-sendotp-route these type and choose according to your use.
SMS menu will open the configuration for the channel, there are some required parameters to use the service, url, auth_key (from the msg91), sms sender and route, fill the the details from msg91 account you can find the settings in api menu in left.
Check the Send sms from the general settings as true ans set the preferred channel,this will be default channel for sending sms.
To send the message to partner open the partner form and check mobile number is filled and valid, from the the action menu click on the Send message button to send message via preferred channel, odoo default sms menu is also active if you have odoo service subscription one can use default service.
Type message in the pop-up area, length is limited in some messages please check with the service provider.