When i use the Tag to push name as "$BUILD_TIMESTAMP" then it succeeded to push tag

but add one more variable such as "$BUILD_TIMESTAMP_$BUILD_NUMBER" then $BUILD_TIMESTAMP did not assigned correctly only $BUILD_NUMBER assigned to tag name

i want to use that name as "20190901_10" but result is "$BUILD_TIMESTAMP_10"

is there a way to use jenkins variable such above tag name?