Welcome to the forum, Liam, and sorry you’ve had to wait a couple of days for an answer.
You can’t tie directly from a note in a slash voice to a slash region, as you’ve found. So you have a couple of options. One, which I don’t think is really ideal, would be to select the 16th note in the slash voice and activate the ‘Laissez vibrer tie’ property, then drag that tie out to the appropriate duration in Engrave mode.
The other, which I think is better all round, would be to move the start of the slash region backward by one quarter, which you can do by selecting the circular handle at the start of the slash region and typing Alt+right (or indeed by dragging it with the mouse). You can then replace the quarter rest at the start of that bar with a quarter in the slash voice you were using at the end of the previous bar, and tie it to the previous note.