I im not sure how to explaine it (see screenshot). I'd like to create a shape (f.e. rectangle), where the fill of this shape fits exactly into the curves of the wavy stroke when aplly it. Without using Illustrator f.e. copy-paste etc.. I need this to be scaleble and 100% in indesign.
That Wavy Line stroke built-in to InDesign! It doesn't behave like anything else in InDesign. Joining paths will (unfortunately) not work in InDesign. Your only alternative is to draw it in Illustrator; then copynpaste it (using AICB) to give you a frame in InDesign. In Illustrator, you have the advantage of using a wavy line generator effect. Build it to size, then expand it. Then copynpaste it to InDesign (preferring AICB, that is Adobe Illustrator Clip Board) to get it over to InDesign as a simple frame.
(for those terrified of the Pen tool, but would like to learn, I have a downloadable lesson file that explains absolutely every facet of how to use the Pen tool)