[TYPO3] direct mail template

[dave typo]

I am trying to apply an html template to my direct mail newsletter,
but I cannot select a template from the Page Template Structure
dropdown menu.

I figured this is because the newsletter page is a subpage within a
sys folder, but several tutorials online explain that I need to do
this step. How can I enable this to work? I add a template to the
page, but I cannot select a template structure.

It also hasn't recursively adopted the template from previous pages.

so how can i apply a template structure when the drop menu doesn't
display any of my TS's?

Let me know,

Thanks,

-Dave