Don't think you can. I open the coin door and add credits. Some people rig up an external button to add the credits with the coin reject button, but I've never tried to do that. Just as easy to open the coin door once in a while.

No freeplay in the factory software

Add a switch to the reject button like shock_me says, or drop in a replacement MPU like Altek which adds freeplay. There is also a homebrew ROM update here (http://www.pinball4you.ch/okaegi/pro_soft.html) that adds freeplay, but I can't vouch for it (my attempt to install it didn't go well, so I abandoned it...)

Set the replay value really low (10,000 points or similar). Set the dip switches to max of 40 credits and then add some credits to the game.

You can do the ROM swap (it works, but you need to re-jumper the MPU board), or you can stack a credit switch under the Start button and that way, a moment before you hit start, you will be adding some credits.

In the meantime, you can set the MPU switches to give you 15 credits per coin.

Home ROM is the way to go. Adds a couple other small features as well.

I did the free play ROM on my Flash Gordon. Definitely the way to go on a good working board. Still get rewarded with the knocker when you earned it versus 10k points. If the board has corrosion, Altek is definitely an option. I read they used the same free play ROM code.