Extending Jira Time Tracking

Hi, I’m new to Jira development.
I’ve been reading this (https://developer.atlassian.com/cloud/jira/platform/modules/time-tracking-provider/). I’ve selected my customer time tracking. Now the Work log action buttons disappeared, which is fine. But how do I add my custom time log page?
My goal is to add my custom fields just like what this add-on did ( https://marketplace.atlassian.com/apps/1219563/moco-time-tracking-for-jira?hosting=server&tab=overview). Where they added their custom fields in the time log entry page.
Pls somebody point me to where I can accomplish this.
Thanks in advance