I just did it this morning with Edge. Had a DH only player I wanted to play 1B.
Select an Optional spot in the draft center and then search for the player you want.
Go to the URL in the address bar and change the spot= to whatever position you need. C=1, 1B=2, 2B=3 and so on...I've pasted the URL with the spot= bolded so you can see where in the URL you need to change it.
https://www.whatifsports.com/mlb-l/themewiz_playersearch.asp?pg=-1&spot=14&spotfilled=0&themeid=-1&dtid=-1&repid=0&bl=0&PreviousSearch=0&cashleft=-412551&uwl=0&ubl=0&dlcl=0&sstype=0&btnSave=false&SavedPlayerSearches=1&ln=&tn0=-1&fn=&PosList=15&mingames=0&secpos=on&ssList=&seasonmin=&seasonmax=&ob0=2&salmin=-99999999&salmax=99999999&ob1=0&arm=0&fls=0&SeasonType=0&max=50&Awards=0&StatList_1=6_6&StatMin_1=&StatMax_1=&StatList_5=6_47&StatMin_5=&StatMax_5=&StatList_2=7_13&StatMin_2=&StatMax_2=&StatList_6=7_50&StatMin_6=&StatMax_6=&StatList_3=7_19&StatMin_3=&StatMax_3=&StatList_7=2_18&StatMin_7=&StatMax_7=&StatList_4=5_29&StatMin_4=&StatMax_4=&StatList_8=7_37&StatMin_8=&StatMax_8=