Since it’s 18 years down the line still not possible to make the “Work description” mandatory when logging time with the built in Time Tracker (https://jira.atlassian.com/browse/JRACLOUD-8751), how could one create this validation using a Forge app (or another technique)?
Since an app is typically sandboxed it’s not possible to just inject JavaScript that would attach to the button click event.
I think one common way in a connect app would be to make your app a “time tracking provider” then your app completely provides the UI for creating / updating / displaying work logs. Obviously this is a lot of extra work if you just want to validate this one particular field… Its also not actually possible in Forge yet, see: [FRGE-619] - Ecosystem Jira
Obviously there are other third party apps which have this type of functionality - but might be again overkill if you just want validation on this one thing.
So basically the only current option is either buy or build a connect app that implements a full UI for adding / updating / viewing work logs.
Thanks @richard.white
Funny enough, I have already been commenting on FRGE-619 
I have developed a functioning Forge App that runs in the Context section, that does time registration including the worklog entries, but it’s not a replacement for the built-in one (as it cannot be replaced). Unfortunately, the client would however prefer to use the built-in one with this extra validation of the Work description.
Hi,
If you want to use your Forge app and disable the built-in time tracking functions, you can install free “WorklogPRO companion app” from the marketplace. This app doesn’t provide time tracking functionality. But after installing it, Jira allows you to select this app as time tracking provider and disables all the built-in time tracking functionality. We have developed this to disable built-in time tracking when using our “WorklogPRO - Timesheet for Jira” application. But you don’t need to use WorklogPRO.
If you want to use a 3rd party app, WorklogPRO allows you to make description mandatory in addition to all sort of different validations..