This is iOS where there is no MouseDown or GotFocus events 
https://documentation.xojo.com/api/user_interface/mobile/mobiletextcontrol.html
I have been told declares are necessary for this basic functionality. I’ve searched a little but haven’t found anything except how to force the focus to the control with becomeFirstResponder. I found this thread suggesting the use of one of those “iosLib” components, however I’m not sure if those libraries were updated for the Mobile* prefix controls.
There is a feedback case from 2014 requesting the GotFocus event. <https://xojo.com/issue/36429>