This archived repo provides the XMTP Inbox iOS app, which is no longer supported.
For an alternative solution, see the example app provided by the xmtp-ios SDK.
Add your Infura Key as a build configuration setting in a file called Secrets.xcconfig in Xcode.
File -> New -> File... -> Configuration Settings File -> **Secrets.xcconfig**
Then add the following as a static environment variable.
INFURA_KEY = {REPLACE_WITH_YOUR_KEY}
If you do not have an Infura Key, you can follow these instructions to get one.