how to make a tuplet like this

You can't make a triplet with 3 16th notes and an 8th note. It is not uncommon to have a 3 16th note triplet followed by an 8th note. See the attached file.

You can do it with a fifthlet as seen in my second measure.

See my picture, you can do that, but it is not a triplet on the whole time, the triplet is only on the 3 16th notes , it gives an half time , and the eight note gives the other half time.

To do that, select the measure, if it's empty, clic the whole rest, click 5. Click the quarter rest where you want your triplet, enter 4, it gives 2 eight rests. Where you want your triplet of 3 16th notes, click the eight rest, enter CTRL+3, it's gives 3 16th rests, enter the 3 16th notes, . On the second eight rest, enter 4, (not necessary) enter the last eight note

Or, to enter this all in one step:

1) enter note inptu mode
2) select eighth note duration
3) press ctrl+3
4) enter your three sixteenths
5) select eighth note duration
6) enter your eighth