Create a DHT node for a new network

I am trying to build a DHT network for an Android messaging app. The DHT shall store messages in case the receiver is not online, so that the receiver can poll DHT for messages when back online.
I was already able to create a bootstrap node with a static address and also a DHT node.
When I try to put a value I get an error: invalid record keytype
I found out that I have to create an own validator, but was not able to find a sample.

Also I need a sample to put and get values on the client side.

Thanks a lot

Edit: Maybe I should mention that I am working with Go