Being new to TYPO3, I'm interested in finding out, how I can add a TYPO3 page type in the backend on page creation programmatically.
As you can find in the manual you can set a default value for each field of any table with Page-/User-TSconfig.
general:
TCAdefaults.<table>.<field> = <value>
specific:
TCAdefaults.pages.doktype = 123
(replace 123 with your doktype)