how to create a half curvy/half rectangular shape in indesign?

hi! i've been trying to figure this out, i think i knew how to do it once then lost those notes and i need help figuring it out again. i can't figure out the right words to describe this to get the right results in google, and i've tried pen tool tutorials but it's been taking me a while to find this exact point in the tutorials. basically, i want an image with a curvy border that's flush with the side of a page. so i would need to create an image mask with a shape that's part rectangular to align with the edges of the document, and part curvy. i'm having trouble creating the shape.

for example, a shape like this (outlined in red):

I would normally just have the shape hang off the side of the page, but I can't do that in a situation where i have something like a trifold brochure with multiple attached pages.

could anyone explain how to do this, or provide a link to a demonstration?

thanks!