You can mark text as italics in any place by surrounding it with \it ...\it*.
\it ...\it* is an example of character-level markup (like a character style in MS Word or other text editor). These always need to be opened and closed. In footnotes, you may get away with not closing it because it is followed by the next character markup like \ft or \fr, but in normal text it does need a closing marker, or else the italics continue till the end of the paragraph.
A note about spacing: the space after the \it marker is part of the marker, so in order to have a space before the markup, there should be a space before the marker. The space after the \it* marker is not part of the marker (i.e. it is formatted as a real space), so there doesn't need to be a space before \it. This allows you to have \it* marker before punctuation. For example:
Markup: \p Here is a properly formatted \it keyword\it*.
Result: Here is a properly formatted keyword.
Markup: \p Here is a wrongly formatted\it keyword \it*.
Result: Here is a wrongly formattedkeyword .
Another note: \it ... \it* indicates a specific formatting; it doesn't tell anything about the function of the text in italics. In general, it is encouraged not to use formatting markers, but functional markers instead. For example, the keyword could be a transliterated word (from Greek, English etc.), in which case you use \tl ...\tl*; or a Bible book name, in which case you use \bk ...\bk*. The available markers are listed in this section of the USFM guide: https://ubsicap.github.io/usfm/characters/index.html#. That being said, there are cases where no suitable functional marker is available, so direct formatting is the only option.
Paulus