https://shulchanaruchharav.com/halacha/not-to-be-dressed-sloppy-tucking-in-the-shirt/
The custom in Lubavitch:[9] The original custom in Tomchei Temimim of Lubavitch was to wear the Tzitzis on top of the clothing. However, after the Rebbe Rashab negated this practice [see above] the custom became to wear a long jacket, which would thus cover the Tzitzis. Later the custom became to wear short jackets and tuck the shirts inside the pants.[10] It is only in more recent years that some Bochurim became accustomed to wearing the shirt out. As stated above, this should not be done and Bochurim are to look presentable and tuck their shirts into their pants.