Little Helper Custom Plugin that creates a new custom field

Hi guys,
To start coding in Mautic for myself, I want to make a small custom plugin.
It should do:
Fetching the value of two custom fields after form submission and putting the data together into one new custom field.

Like that:
1.Form gets submitted via website
2.Data:
Field One:
Type: Time
Value: hh:mm:ss

Field Two:
Type: Date
Value: yyyy-mm-dd

From there the plugin should get these two values and combine it into a new date/time type custom field with a value “yyyy-mm-dd hh:mm:ss”

any suggestions how to begin?

I would appreciate any ideas or hints whats needed in terms of services/events/mautic coding related stuff. It is really hard to get going only with the documentation.