Fill in a textbox according to checkbox and option selection (don't even know if i'm creating it the best way

On a userform I am trying to populate a textbox with cell value according to a checkbox and optionbutton combination.

The bigger picture is i'm trying to create a macro that will automatically create emails.

Firstly I have 7 checkboxes (Applications) , one could be ticked or a couple could be ticked. Then it is the type of email I would like to create, for this I have used optionbuttons.

Not necessarily after code (though that would be helpful) but just need some guidance on how to code it as I have written and then re-written so much code and the last one was way over the top for what I needed.

Any advice much appreciated