When asking here, it helps when OP mentions OS name, LO version and save format. Consequently, I answer only in general terms. Adapt to your specific case.
All I could find in the huge Unicode repertoire is:
- U+0352 COMBINING FERMATA ͒
- U+1D110 MUSICAL SYMBOL FERMATA 𝄐
- U+1D111 MUSICAL SYMBOL FERMATA BELOW 𝄑
ERRATUM: Warning! in the initial version of this post, hexadecimal for last too was faulty!
The first one behaves like an accent and needs a “supporting” character to be visible. The latter two don’t live in the Unicode Basic Multilingual Plane. Expect then problems with common fonts which very likely don’t provide them. They display on my Fedora Linux box but due to the system substitution engine, I don’t know from which font they were extracted (I search for them in the OS not in Writer).
Inserting them in Writer is quite easy: type “U+…” (without the quotes) where … stands for one of the hexadecimal codes above, immediately followed by Alt+X. If they are not present in your current font or a substituted associated one, you’ll get the “missing symbol” for the font, usually an empty rectangle.