Draw the triangle part first - the draw the rectangle.
Overlap them where you want them.
Use Pathfinder Add
And that should be it.
You can do this by using the Auto-Size option on the text frame and a separate object (or 2) for the arrow/pointer.
Create a text frame that auto sizes away from the direction of the pointer. Make sure the stroke on the text frame is aligned to the outside. Draw a pointer with the Pen tool and give it the same stroke and fill as the text frame.
Then you either have to carefully align the endpoints of the pointer to the edge of the text frame or if you want perfect-looking joins, have the endpoints of the pointer overlap the text frame and cover them up with another frame filled with the same fill color as the text frame.
You may have to experiment with the position and size of this obscuring frame so it doesn't become visible when the text frame resizes. Here's a snippet you might be able to use as a starting point. Dropbox - resizing_speechbubble.idms
That's it. I changed your snippet in the way i need it. Thanks!!
Here's another way I found:
1. Draw a speech bubble. Mine is a rectangle with rounded corners and a triangular pointer added with Pathfinder > Add
2. Drag out a frame the same size as the speech bubble. Select the speech bubble and Copy; then select the empty frame and choose Edit > Paste Into...
3. Alt-Drag the frame with the pasted speech bubble to make a copy, then crop one copy to leave only the top of the bubble showing, and crop the other copy to leave only the bottom.
4. Drag out a text frame and insert a table consisting of 1 column, 3 rows. Set the text frame to Autosize > Height Only.
5. Set the stroke/fill of the top and bottom rows to none, and style the middle row to match the speech bubble, (in my case a white fill and 2pt stroke; left and right).
6. Anchor (paste) a copy of the speech bubble top in the top table row, and a copy of the speech bubble bottom in the bottom row.
Getting the 3 parts to match up with is where you just have to work on it until you get it right. Use the positioning tools in Anchored Object options and the column width setting in Cell options to line everything up.
Enter your text in the middle row. (Hey, look at that...a valid application of Comic Sans!) With the Cell Height set to an "At Least" setting, the cell will expand to fit whatever text you enter, pushing the the bottom row down, with the text frame auto-sizing to keep everything in play...