Dankie by voorbaat
However, depending on the context, other options might be more appropriate:
* Dankie vir jou hulp vooraf (Thank you for your help in advance)
* Dankie vir jou tyd vooraf (Thank you for your time in advance)
* Ek waardeer jou hulp vooraf (I appreciate your help in advance)
It's important to consider the specific situation when choosing the best translation.