Once you own Bitcoin "on-chain" there are several methods to transfer them to an app that supports the Lightning Network.
*This is not a recommendation of any service and is for information/educational purposes only. These methods are not suitable for holding significant amounts of Bitcoin and should be treated like cash in the wallet. All of the above methods involve paying a fee. Please pay attention to the amount that will be shown in the application at the time of the transaction. The best method to hold Bitcoin on the Lightning network is via your own Bitcoin/Lightning node.