GIRM is silent because it is totally inappropriate to sing a hymn (prayer) after the liturgy is finished.
...because if it were necessary to complete the liturgy, it would have been documented...
…can someone tell me the codes on this forum for block quotes…
28. The distinction between solemn, sung and read Mass, sanctioned by the Instruction of 1958 (n. 3), is retained, according to the traditional liturgical laws at present in force. However, for the sung Mass (Missa cantata), different degrees of participation are put forward here for reasons of pastoral usefulness, so that it may become easier to make the celebration of Mass more beautiful by singing, according to the capabilities of each congregation.
These degrees are so arranged that the first may be used even by itself, but the second and third, wholly or partially, may never be used without the first. In this way the faithful will be continually led towards an ever greater participation in the singing.
36. There is no reason why some of the Proper or Ordinary should not be sung in said Masses. Moreover, some other song can also, on occasions, be sung at the beginning, at the Offertory, at the Communion and at the end of Mass. It is not sufficient, however, that these songs be merely "Eucharistic"—they must be in keeping with the parts of the Mass, with the feast, or with the liturgical season.
65. In sung or said Masses, the organ, or other instrument legitimately admitted, can be used to accompany the singing of the choir and the people; it can also be played solo at the beginning before the priest reaches the altar, at the Offertory, at the Communion, and at the end of Mass.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.