THISFORM.TEXT15.VALUE= PADL( INT(( AJK29 % 86400 ) / 3600 ), 2, '0' );
+SPACE(1)+ALLTRIM('-')+SPACE(1) ;
+ PADL( INT(( AJK29 % 3600 ) / 60 ), 2, '0')

When you write into the textbox value,
VFP reset the TextBox SelStart property.

you can ( with variable lenght text, the solution is more complex ) try this:

WITH M.THISFORM.TEXT15
   selStart = .SelStart
   .value = PADL( INT(( AJK29 % 86400 ) / 3600 ), 2, '0' );
+' - ' + PADL( INT(( AJK29 % 3600 ) / 60 ), 2, '0')
   .SelStart = m.selStart
ENDWITH