Where is the text inset control? I’ve looked around in the menus but can’t find this.
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Duplicate at 283200/how-can-i-change-the-margin-of-text-wrap-against-a-text-box
Text boxes are Drawing objects so have limited text functionality (no text styles), think labels for drawings. Better to use Frames, Insert > Frame > Frame interactively.
If you insist on text boxes, and probably having to manually craft each one, then double-click inside your textbox, select your text.
Open the sidebar (Ctrl+F5, press the sequence again if it doesn’t open the first time).
In the sidebar click on the Properties icon (2nd from the top), under Indent you can set the space before the text, the space after the text and a first line indent.
Cheers, Al