To set a sender id as the default sender id, Go to SMS Settings>Basic Settings here, once you scroll down to the sender id section, you will find a button named as "Set Default". By clicking on this button, you will be re-directed to a page where you can select the appropriate sender id from the drop down and select the appropriate profile from the available profiles section and save the same. Refer the below screenshot for the same:
The user profile which you select while setting the default sender will always see the selected sender id as the default one whenever they click on the Send SMS button.
To know more, please visit the below link:
Feel free to revert if you need any further assistance.