As of right now it is not possible to choose between using a warlock spellslot or for example sorcerer spellslot to use in casting. This disencourages multiclassing into warlock especially because for most spells and feautures the standard class spellslots are used before warlock spellslots (meaning the ones you don't regain on a shortrest). Even when clicking the warlock spellslot and clicking a spell or other ability it first utulises non-warlock spellslots if it isn't a warlock ability.
I don't know how easy or difficult this would be to implement but I believe the best way to change this would be for warlock spellslots to occupy their own subsection when selecting
a spell.
Perhaps having them be displayed either to the right or left to all your other spellslots, or have them be displayed below the appropriate spellslot of the level they are.
I hope this get's seen as there are obviously many posts on this board per day, but I think this would make a major improvement to warlock multiclassing.
If any devs and up seeing this, thanks for this amazing game :3