¿Agárralo? ¿Sosténgalo?
¿"Agárralo así"?
The main verb would be agarrar which applies to both: establishing the hold and maintaining it. There is also "agarrarse" which would be "to hold on to".
Asir might be a perhaps-overly-sophisticated and perhaps-acceptable synonym that I personally wouldn't use.
If you try to grab a hold with a very dynamic (i.e.: fast) movement and get close to it, perhaps even touch it, but fail to close your hand around it, that's "manotear" or "dar un manotazo" (according to the rules: "tocar" and you need to keep your hand on it for three seconds for it to be "agarrar").
If you do reach the hold with enough time to put your hand on it but it slips off your fingers, or whatever, we say things like "no la puedo agarrar" ("la" being "la toma", the hold) or "no la puedo agarrar bien" or "no me quedo" or "se me zafa" (This last one might be Argentinian) or "se me patina" or "se me va".
Example:
Mientras estás agarrado de la regleta (crimp) azul, usas el piecito (little pie, little foothold) rojo, y con la izquierda agarras la amarilla grandota, que es buena, y mientras estás agarrado de la amarilla, cambias de pie y recién entonces vas a la regleta roja. Si no cambias de pie, no te vas a poder sostener de la roja, te vas a abrir como una puerta (barn door)
I think "agarrar" would be more appropriate in that context. "Sostener" is normally used to say "to hold".
If we consider the climbing hold as the subject then we could say that it is holding you ("La presa de escalada te sostiene"). But when it functions as the object, I think it is more appropriate to say that You are "grabbing" the climbing hold ("Tú agarras la presa de escalada").
As said, I think agarrar would fit the best. Also note the massive use in Spain of coger, with same meaning, that you should not use in Spanish America, as it has a totally different meaning (definition 31).