You can do this two different ways
Method 1. Create an autocorrect so that you type ofc and then add a space to have it automatically change to of course.
Method 2. Create 'of course' as a Building Block with the name ofc so that you can press F3 after typing it to force the change.

The first method would be less work to use in practice. Are you sure you want the Building Block method?

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Andrew Lockton
Chrysalis Design, Melbourne Australia