Issuing/receiving challange

I have received two challanges where the opponent is playing white. However, when I respond, no opening move had been made.

Is the proper procedure, when issuing a challange as white, to already have made an opening move?

Other than messaging my opponent, is there some other way to respond to the challange?

There is no other way to respond then by messaging the opponent. What you can also do is delete the game if you think that the opponent isn't going to move.

Check the TO period first. Perhaps the opponent isn't able to move yet and you just have to be patient.

edit: when issuing a game as white you don't have to make the move untill the challange is accepted and the TO and TB starts to work.