This can be done by modifying the boot.tpl code.
In the boot.tpl file, just before the call to LoadConfigInit, write a 1 to the corresponding bits of the Port data registers (PRTXDR). For example, to initialize P1 and P1 to a HIGH state after power up, add the following code in boot.tpl before the call to LoadConfigInit.
mov reg[PRT1DR], 0xA0
When the pin's drive mode gets configured in LoadConfigInit function, the pin state will change to HIGH.
Further, note that it is always advisable to add a comment in the beginning of main.c about the modification in boot.tpl. This is because, in the future versions of the designer, the Boot.tpl file might be updated and if the user decides to upgrade to the new Boot.tpl file, then the modifications done to the Boot.tpl file will be lost.
So, it is always advisable to have a record of the change in the main.c file. A solution for that is to keep the drive mode of the pin as High Impedance Analog in device editor view and in the application code area, assign a default state to the pin and then configure the drive mode.