Assuming you're referring to the type of text box you insert with Insert>Text Box, those cannot be part of the text flow in a form created with Legacy Form Fields.
But Legacy Form Fields are old tech and it's easier to create a form using Content Controls instead. Here is Microsoft's page on creating fillable forms: Create a form in Word that users can complete or print. The Windows tab covers forms created with Content Controls, the Mac tab show how to create a form with Legacy Form Fields.
While it's possible to use both Legacy Form Fields and Content Controls, your life will by easier if you choose one of those and stick with it for the whole form.
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