How can I code a slur??

A slur is a "bent note".

For short slurs, I do this with the sequence of "strb C". This puts a 32nd note B in front of the C and you get the "slur".The note is so short that you can imagine that it's slurred.

For longer slurs, there is also the "echob C" code which puts a 16th note in front C, but the 16th note is long enough that you can't imagine that you hear a slur. Either way, it gets the point across on the printed score.

Be aware that may slurs start with a gracenote, so "gg strb C" is a pretty common approach.

Perhaps a future version or some other software might include "bslurc" or "lgslurla" codes and implement a sound command called "portamento"(?), but, to my knowledge, this capability has not been designed into the current version of any bagpiping music program.