I do it this way make the mtext then get the entlast using vla-put-textstring add the underline code to front of your text string.
"abcde" underlined "\\Labcde"
But text string could look like this "{\\fArial|b0|i0|c0|p34;abcde}" this seemed to work "{\\L\\fArial|b0|i0|c0|p34;abcde}"