Enter key.... carraige return. Will push text to the next line. Works like any other text editor.
I think you need to create a sketch, sketched symbol, or something similar and use the Text tool within that environment before you can use the Text Box option. For some reason the standard drawing text tool and leader text tools both lack the ability to define a text box.
Within a sketch you can use the Text Box button in the text editor, or right-click on the text and choose Text Box. Then you can define the length of the text box with a dimension or just click and drag a corner to eyeball it.
If you have this need often, it's probably worth creating a sketched symbol to use, and just include a leader when placing it.
Adding the Text Box option to the standard drawing text tool would probably make a good feedback request:
I hope this helps.
Best of luck to you in all of your Inventor pursuits,
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