When viewing your 1st team squad list, Next to your team name near the top of the page it says "Senior Squad." There is a drop down arrow right next to it. Click it and you shall find "Reserve squad" and "U-18 team" options appear.
Choose Reserve squad, and then click on "Fixtures".
And from there click on "arrange friendly" and there you are!
Follow same steps for U-18 team but ofcourse picking U-18 instead of Reserve this time.