Hey! From the Monzo app, tap the transaction you want to dispute, scroll down all the way, and tap Something wrong? Get help. You’ll get a help screen to read, tap one of the options, and you’ll either go into a series of questions, or be directed to Customer Service. 
Hope that helps!
We would far prefer if people started from the transaction screen. 
In an increasing number of cases, you can resolve issues within the app without speaking to customer support. Even if you do get put through to in-app chat from there, it will be tagged with the transaction already there so we don’t have to ask you to identify it.